I've been having some trouble with my clutch.I've been experiencing some hard peddle off and on and today I was downshifting and the peddle stuck down:confused: .I was a little comcerned,is there anything too unusural about this with the stock setup?? Thanks

Anyone?

Common issue. From reading the forums, I think you will find that GM currently has no real solution. It seems to be related to a slightly slipping clutch. Opinion seems to be that an upgraded clutch kit from SBC takes care of this. Also, a couple of vendors are working on a SMF and conventional clutch disc to address this issue as well as offer more holding power. Some rumors seem to indicate that GM may have a fix in the future, as they currently de-tune the Duramax when coupled to the poorly designed clutch setup. GM will need to resolve this to remain competittive with Dodge in this area.

My .02

Further searches on this board will yield a great deal of reading.

Anyone?

Problem here is trans overheating (ZF650) causing fluid to boil
GM rep finaly ageed now they are putting on a trans cooler??????-:t

The hard peddle is not caused by heat from the ZF but by the clutch slipping and inducing heat into the clutch hydraulic line.

The hard peddle is not caused by heat from the ZF but by the clutch slipping and inducing heat into the clutch hydraulic line.

Could the hot trans oil be causing the hydraulic oil to to heat up and cause the pedal to become hard? Isnt the slave cylinder right near where hot trans oil could heat the hydraulic oil up? I find it hard to believe that this many clutches slip at highway speeds. I think almost everyone ive heard talk on here has said that they're clutch gets hard after long stretches of highway driving.

I could be wrong, but do you really think GM sells trucks with slipping clutches from the factory? Mine has gotten hard from highway driving since it was brand new, well before i added my juice. It doesnt get any worse unless i can actually feel the clutch slip.

The hard peddle is not caused by heat from the ZF but by the clutch slipping and inducing heat into the clutch hydraulic line.

stealer put a new pedal kit and tranny cooler in and the pedal still got hard. replaced with CPMac's SMF and dual disc clutch. pedal never got hard (at least like stock) even when it slipped and wiped off the front side pad on one of the discs.

tranny temp, around here, takes hours on a highway to get up to 200*F. My guess is the problem is with the DMF creating extensive heat for some reason.

When the pedal got hard, I would stand on it and it would sink down and stay on the floor until I pulled it up with my toe. Then it would return to 'normal'.

If you think that the ZF doesn't get hot put gauge on it, I have @ 200+ peddle gets hard @250+ peddle goes to floor and stays. Yes the fluid does boil. The clutch is still like new, on third flywheel and this one is groaning. I bought a new clutch/presure plate when the first flywheel went bad and dealer replaced original each time.

I must be lucky...I never had the hard peddle issue. Now every now and then, the top 2" to 3" of the pedal travel is real spongy...but it will clear up after driving for 10 to 15 miles. This is by far the weirdest clutch setup that I have ever used.

MaxRock

I must be lucky...I never had the hard peddle issue. Now every now and then, the top 2" to 3" of the pedal travel is real spongy...but it will clear up after driving for 10 to 15 miles. This is by far the weirdest clutch setup that I have ever used.

MaxRock


Mine has been spongy all the way to the floor -:t they told me it was the heat.

TOM;
how long does it take for the tranny temp to get to 250*F ? are you towing when this happens?

suggest you get a tranny cooler installed. mine keeps the temp no more than 215*F - $tealer installed it to stop the hard pedal but it didn't. I do have a tranny temp gauge (Faze) and it took several hours on the highway to get all the way up. I was not towing.

I have a ZF S6-650, and I too have had the pedal problem from boiling clutch fluid. My problem occured when I was towing my gooseneck trailer on a hot 95F August afternoon. I was pushing a headwind, and going about 75mph at the time. Never detected any clutch slippage though.

A couple of things I did to my truck to solve the pedal problem was to move the clutch hydraulic hose to the outboard side of the brake lines. By moving it to this area, it gets some cooling air, and moves it away from the hot engine. I figured the heat from the engine would affect the brakes before it got to the clutch line in this position.

Also, I drained as much of the old fluid as I could from the system, and replaced it with a high boiling point DOT4 fluid. Valvoline Synpower was what I used. At the time, it had the highest boiling point that I could find.

Since doing these two steps, I haven't had the problem.

I have a ZF S6-650, and I too have had the pedal problem from boiling clutch fluid. My problem occured when I was towing my gooseneck trailer on a hot 95F August afternoon. I was pushing a headwind, and going about 75mph at the time. Never detected any clutch slippage though.

A couple of things I did to my truck to solve the pedal problem was to move the clutch hydraulic hose to the outboard side of the brake lines. By moving it to this area, it gets some cooling air, and moves it away from the hot engine. I figured the heat from the engine would affect the brakes before it got to the clutch line in this position.

Also, I drained as much of the old fluid as I could from the system, and replaced it with a high boiling point DOT4 fluid. Valvoline Synpower was what I used. At the time, it had the highest boiling point that I could find.

Since doing these two steps, I haven't had the problem.

How did you drain the clutch fluid?

Well, there is no real way of draining ALL of the fluid with out removing the transmission. (B.S. Kraut transmission) You can get most of it by draining the master cylinder, and back bleeding the slave cylinder.

What I did was to drain all of the fluid from the master cylinder, and then just keep pumping it up, and bleeding it until it runs clear. (the fluid turned black when I had my episode) I would have my wife pump it up and then hold the pedal down. When you open the bleeder the clutch's pressure plate springs force the piston back in, and this returns the fluid back up the line.

There is NO bleeder on the slave cylinder. Only on the master cylinder. So the bleeding process is a long slow boring deal.

I have thought about calling ZF North America and seeing what their thoughts were, but my truck is an '02, and since it hasn't done this except for 1 time in the summer of '02, I had kind of forgotten about it, until I seen a thread on this same subject over on GM-trucks.com.

Thanks Engineer for the info...this is the only way I could think of flushing the fluid. I put in Valvoline Syn DOT 4 when my slave cylinder started leaking. This may be the reason I do not have the hard pedal problem.

MaxRock

I went and dug up my old bottle of Valvoline Syn Power DOT4 brake fluid yesterday. The bottle claims a dry boiling point of 504F. Some other brands of DOT4 brake fluid are less than that. The minimun legal claim to get a DOT4 rating is around 440F, I belive.

I should tell you guys, since I can't seem to get a sig line to work, that I have the 8.1L gasser, and not the D-Max. Since my exhaust is routed on both sides of the engine, and the D-Max's is on the right, I assumed that the D-Max wouldn't have the problem. I belived my problem stemmed from the close proximity of the exhaust on the 8.1 to the clutch fluid line was where the source of the evil was. But if the D-Max is having problems as well, I have to stand corrected on that theory.

Maybe the OEM fluid is just not up to the task for either application. Whatever the case, I hope you get your truck fixed to your satisfaction. IMHO, Chevy builds the best trucks you can get. But even still any machine will be subject to problems.

As I said earlier I had the problem where the pedal stayed on the floor one time during an extreme useage condition. That was three years ago, and the problem hasn't occured since.

Please keep me updated on how things work for you. If you try my fixes, and still have problems, I would appreciate you letting me know. Maybe we can work on another fix, or contact ZF North America directly.

I spoke with a clutch mfger about this problem. He was aware of this problem, though he did not have a dmax to investigate, he had discussed this with others. His opinion was that flywheel/clutch/pressure plate design was allowing a push back on the slave cylinder which pushed pack on the hyd fluid. He believed there was an in-line check valve that was getting locked-up as a result. I looked at the line last nite and it appears that at the top of the hyd line there is black plastic doo-dad between the top of the line and the master cylinder that has identicaly connectors on each end held in place by a wire "c" shaped c-clamp. My guess is this is a check valve.

opinions?

Deadeye-

I have seen your plastic doo-dad, and just assumed it was an assembly line quick-connect fitting.

May require more investigation on my part.

I only experience the hard pedal issue during long stints on the freeway, happens basically everytime i down shift after gettting off the freeway...guess I am just used to it now. However, it has never stuck to floor on me even when I was pulling extremely heavy loads. I don't recall feeling hard it the one time I literally smoked my clutch to the point where I had smoke in the cab...hmmmm?
It seems logical the check valve ball could get stuck and the extra fluid it traps would cause the issue. Anyone know what the ball is made of, if it is heat related perhaps the heat is causing the ball to expand in size which is typical with plastics and steel causing the ball to stick?? I don't know, just some of my rambling thoughts.

This is good info fella's. The check valve may be the issue. Next thing you know Super Diesel will be making industrial strength check valves for the ZF!!!

MaxRock

This is good info fella's. The check valve may be the issue. Next thing you know Super Diesel will be making industrial strength check valves for the ZF!!!

MaxRock

what a great idea :ro) oh, BTW we man need some marbles for this . . . you got any :muahaha:

what a great idea :ro) oh, BTW we man need some marbles for this . . . you got any :muahaha:

Sorry...can't help you on this one...I've lost all of my marbles!!!!

MaxRock

I've had it apart and all it is a PLASTIC little rod that you can see when you take it apart at your dod-dad. Had it apart for my clutch replacement. It seems to keep air out of the line and fluid in when you disconnect the two lines. I am guessing that the rod just pushes some "marble" down to let fluid though when you push the two back together. My .02:D

I kinda for got about this thread I started.I get hard peddle any time I'm pushing the truck,even just for a couple minutes.Empty only,I've never noticed it when loaded.The time it stuck on the floor,I was driving around 50-55mph:confused: .

Since I've got my new flywheel, clutch and pressure plate I have not noticed the stiff pedal. I'll keep you guys informed.

mine got hard friday just driving 40 miles in 92 deg. heat and I was not driving hard at all, and this is with a new flywheel, clutch and presure plate.

mine got hard friday just driving 40 miles in 92 deg. heat and I was not driving hard at all, and this is with a new flywheel, clutch and presure plate.

Are those new parts....stock?

I had something strange happen back around X-mas, traveling home to upstate NY. (this was on my second flywheel) After running around 350 mi non stop, I stopped to refuel, and had next to NO clutch peddle !! Fluid level seemed low, so I added..... no change. This continued the rest of the trip (total milage 850mi) The next day, everything was fine?? I've also experienced the hard peddle issue on the original flywheel, and the first replacement. The third one was installed last week.... we'll see, I guess. I really love my truck, but can't stand having velcro on the transmission case . I'm really peaved w/GM, so much so, I've even test drove a dodge.

I think these that SD is building should fix our problems.I'd sure like to get mine fixed.

Are those new parts....stock?

yes all new stock parts back in May.

I had something strange happen back around X-mas, traveling home to upstate NY. (this was on my second flywheel) After running around 350 mi non stop, I stopped to refuel, and had next to NO clutch peddle !! Fluid level seemed low, so I added..... no change. This continued the rest of the trip (total milage 850mi) The next day, everything was fine?? I've also experienced the hard peddle issue on the original flywheel, and the first replacement. The third one was installed last week.... we'll see, I guess. I really love my truck, but can't stand having velcro on the transmission case . I'm really peaved w/GM, so much so, I've even test drove a dodge.


same thing happened to me, I coasted into a rest stop shut off the truck and then put it in gear, came back and clutch peddle went to the floor, had to pump it up to get it to work -:t

I previously posted the pedal sticking had never happened to me....well it happened last night I was hammering on the truck pretty hard with the HJAT on 4 and the pedal started sticking. I will definately be doing some more research on this, hopefully when we get the new clutch kit in the truck we can test this some more. Also Deadeye and I are investigating the check valve as a possible culprit for this. Hard to believe that little peice of plastic is doing the job adequately.

I have never experienced anything like this before. It can be so bad sometimes, I don't think I'm going to make it home. This weekend, I was the event coordinator/race chairman for a local road race, and several times I had run something out to a corner worker on the track. I have very limited time to get off of the road course, so I blow some smoke and high tail it out of there. Clutch is hard as a rock. It only happens to me when I really get on it, towing or not. But this weekend, it would do it in half a lap, or about 1 minute. Where is all this heat coming from in 1 minute? I have noticed it getting worse as the ambient temp goes up. I dont know what to do. Oh, and the check valve doo-dad is just a quick disconect fitting.

I bought a turkey baster at the dollar store and sucked all my clutch fluid out and replaced it with fresh. Just got back from a trip pulling my enclosed sled trailer. It really seemed to help the hard pedal. I'll probably add that to my normal oil or fuel filter change schedule.

Vrruumm, mine used to be way more ambient temp sensitive. But actually this summer it isn't as bad even thought it's been hotter. Maybe my ratteling flywheel keeps the heat out of the clutch.

Jeli,

Do you pull as much fluid out as you can and replace with DOT 4? This may be a good item to add to the preventive maintenance program.

MaxRock

I re-installed my tranny, SMF + dual disc, and new slave cylin yesterday. It is working better than it ever has do to some mods by SBC and some installation changes (with the help of IBDMAX'IN). Learned a few things.

The hard pedal is definately a result of hydraulic oil heat (boiling) and can come from several sources. Ambient temp is partial culperate. Towing in heat, fast, and/or up hill as well as for numerous hours will run the tranny temp way up. The slave cyl is bolted to the front of the tranny (around the input shaft inside the bell housing) with a lot of surface mounting against the tranny case. This can transfer a lot of heat to the hyd oil inside the slave cylinder which is "inprisoned" inside the bell housing and therefore can not get any air to run by it, unlike slave cyl on the outside of a bell housing.

Hauling a lot of weight or using a chip will cause some clutch slippage, though you may not notice it. Certainly, any fast acceleration with a chip, for instance like a juice at lvl 2 or higher, will cause slippage especially when shifting. This heat will be generated inside the pressure plate, inside the bell housing, right next to the slave cylinder. Again, this will cause hyd oil boiling. I know of several members who have had this happen and it has happened to me numerous times, with the tranny still cool (<150*F).

The boiling oil expands inside the slave cylinder and hydraulic hose and expands the slave cylinder against the pressure plate spring fingers and pushes back on the check valve (see post #17). The check valve has a black plastic "ball" (actually a pin about a 3/4" long, looking at the end it is in a "+" shape) with an o-ring and a small coil spring. The check valve is intended to do two jobs. One is to keep the oil from spilling when it is disconnected from the master cylinder. Additionally, when connected to the master cylinder the end of the "ball" (actually an + rod) pushes another check valve form the master cylinder end of the connector so that the hyd oil can flow back and forth between the master and slave cylinder. If the "+ rod ball" is removed from the check valve the master cyl check valve will not be open and pushing the clutch pedal will blow the master cyl off the fire wall.

When the hydraulic oil boils and expands it pushes hard on the "+ rod ball" pin and expands the o-ring and gets it stuck until either you stand hard on the clutch pedal or let it sit until it cools. (probably would take quite a while)

There is really not much that can be done to solve this problem on a duramax other than replacing the oil as engineer suggested. Probably do this regularly. BTW, If you don't have a lot of hoses and other stuff around where the hydraulic hose connects to the slave cylinder just outside the bell housing on the driver's side, there is a way to drain the oil. Carefullly remove the 17.5mm clip pin that holds the connecters together. Since you have not disconnected the check valve from the MC, above, you can safely operate the pedal to drain the system (except for the SC). When you re-connect the hose to the SC carefully examine the alignment of the male/female connectors, to insure that the space where the clip pin is to be inserted, so that the clip pin can be easily and smoothly inserted. The pin clip on both ends of the hydraulic line are the same size (17.5mm) and the same shape as the much bigger pin clip on the driver's side end of the check valve. I would recommend purchasing a couple extra of these before doing a disconnect (of 17.5mm) pin clip. If you loose or break one you truck is shut down until you get another. $tealers don't usually have any in stock.

A tranny cooler is probably a good idea to help reduce the extent of the pedal problem but it will not completely solve it. Any chip will likely cause some clutch slippage (this was my major cause) and will wear the clutch down. This will move the pressure plate fingers closer to the slave cylinder bearing (about the size of a donut) and transfer more heat.

I don't know for sure, but I would guess that a slightly thicker, tougher clutch plate would help reduce the problem. A thicker clutch plate will allow more holding pressure and should reduce slippage. Ditto for a stouter material for the clutch plate pads. (All assuming a stock DMF and PP). If you decide to do this I would consider replacing the SC also. I surely gets impacted by the heat for which it was not (I assume) designed to hold to.

Unfortunately, the GM DMF likely adds some temp to this problem as it begins to deteriorate. How much ???

I have not had a hard pedal problem (due to heat) since the SMF DD kit was installed. This is likely due to the lack of clutch slippage (accept for one time). I also noticed that the PP spring "fingers" which are pushed by the slave cylinder bearing are much thicker/heavier that those on my DD pressure plate. I would assume that this would have a limitation (how much) on heat transfer?

As far as the check valve between the hydraulic hose and the master cylinder is concerned, I can only foresee one action which might help. The o-ring on the "+ rod" which is used as a check valve ball might be a part of the problem. I do not know what the impact on that rubber o-ring is after several very hot hard pedal problems. Perhaps replacing the o-ring (if the hardware store has the correct size) might help? Is there other o-rings made of more heat resistant materials?

I have not looked into the following idea but perhaps some stressed ZF-6 member would be willing? If you look at the check valve connection you will seed that the male connector attached to a ~1/4" steel line from the MC is (IIRC) connected with the 17.5 mm clip pin. If the check valves were removed from both the MC male connector and the check valve female connector (it has to come out from the hyd line side ) this might have a positive result on the hard pedal ? I suspect the check valves are only included for the purpose of stopping oil leakage during maintenance. If someone were to try this I would recommend lowering the fluid in the MC down to "Min" to reduce the chance of fluid expansion causing a problem in the MC ??

If someone is interested in trying this and would like to discuss with me first, feel free to PM me and we can talk on the phone.

Well, I hope at lest some of this info is helpful to those of you dealing with these problems. I am hoping that a reliable replacement kit will be available prior to the end of the year.

Good luck.

Thanks for the information. I too am having the hard pedal problem. I am thinking of changing my SC out just so I know when I get the SBC set up back that everything is new on it so I hope for my life I never have to take this clutch apart again. It's not hard but it still is a pain to work on trucks 40+ hours a week and go home and do it to my own. DId you have to bleed the system when you replaced the SC and line or does it come with the oil in it and all you have to do is connect them? If not please tell me how to do it so I know this weekend when I put it back together.
On the heat topic this maybe dumb do you think there is anything that we could put around the SC like header wrap to keep some of the heat off it or would the lack of space that would be the down fall to that idea?:confused:

D-Farmer;
No, I did not have to bleed the system when I installed the SC. I did add a litlle Hydr Oil into the reservoir. I did not check it but since I know the MC comes with hyd oil the SC probably does also.

I don't think there is any way to insulate the the SC. If there was a gasket for insulation between the tranny case and the SC that might help but it might have an impact on the spacing between the SC "throughout bearing" and the PP spring fingers. If a thin, insulated gasket could be made that might be an interesting experiment. Problem is the PP preload is in the neighborhood of .150" and a gasket might cause the bearing to push on the pp fingers which cause heat, wear, and clutch slip. I will investigaate this and post anything I find.

I just had a discussion with Peter of SBC about several clutch issues. One had to do with the hard pedal issue. He, at one point, considered the fluid heat as an issue but after looking into the pressure plate and DMF these seem to be the real cause. The stock PP has a self adjusting mechanism for the diaphram fingers once it is installed and and the SC bearing hits the fingers. This cannot be viewed because there is not a "window" to view the connection between the SC tearing and PP. However, as acceleration happens, while the tranny and hydraulic oil are still cool, the pedal can get hard. The self adjusting mechanism (appearantly) is a continuous activity that adjusts the position of the diaphram fingers and pushes back on the SC bearing forcing the hydraulic oil to seal the check valve at the top of the hydraulic line. Peter agrees with my suggestion to pull both check valves (see post#36) and see if this reduces the hard pedal problem. Peter has done a lock down of the self adjusting mechanism on the stock PP and it stops the hard pedal for a while. Over time however, the shifting clutch becomes very snappy with a violant engagement. This lock down does increase the PP load on the clutch and reduces slipping, for a wihle. Additionally, the DMF may also be part of the problem as it may also push the clutch and PP diaphram back on the SC bearing as the DMF starts to fail and seperate.

This makes a lot of sense as my aftermarket FW/DD kit has never been hard like the stock. However, the pedal was always tough to push because it's install design did not reduce the diaphram to a more level position, which required more leg strength to operate the pedal as the diaphram fingers were extended back towards the SC throughout bearing. . . (how I explained this clearly)

Anyhow, SD and SBC will have a replacement for the stock system in a few months. In the meantime, try removing both check valves (be careful).

Great info Deadeye, I can guarantee you we will test all of these theories this weekend! I can guarantee the remaining life of the clutch as mine is probably paper thin by now and contributes to this issue. Mine sticks to floor consistantly now on all settings except stock. Let's put it to the test this weekend and see if we can figure this out for all our fellow ZF-6'rs.
We will keep you all posted with our results!

MaxRock, The dealer had mine a bit over max so I filled it to just below max. So far so good. One thing interesting was the rubber boot was full of water. Not sure what to think of that since I the firewall doesn't get wet in the rain and I haven't pressure washer the engine. Must be a fluke.

Deadeye, Your o-ring idea is good. I'd try replacing a known hard peddle with a new o-ring. I'm guessing it is already good for 400 F. Looking at the compatibility chart for DOT 3 fluid the highest temp o-ring with the best compatibility goes to 500 F. I can get samples if we can get an accurate size, inch or metric.

Deadeye you were right the clutch pedal feels so much better now and does not get hard like the old one did when the clutch slips.

I just had a discussion with Peter of SBC about several clutch issues. One had to do with the hard pedal issue. He, at one point, considered the fluid heat as an issue but after looking into the pressure plate and DMF these seem to be the real cause. The stock PP has a self adjusting mechanism for the diaphram fingers once it is installed and and the SC bearing hits the fingers. This cannot be viewed because there is not a "window" to view the connection between the SC tearing and PP. However, as acceleration happens, while the tranny and hydraulic oil are still cool, the pedal can get hard. The self adjusting mechanism (appearantly) is a continuous activity that adjusts the position of the diaphram fingers and pushes back on the SC bearing forcing the hydraulic oil to seal the check valve at the top of the hydraulic line. Peter agrees with my suggestion to pull both check valves (see post#36) and see if this reduces the hard pedal problem. Peter has done a lock down of the self adjusting mechanism on the stock PP and it stops the hard pedal for a while. Over time however, the shifting clutch becomes very snappy with a violant engagement. This lock down does increase the PP load on the clutch and reduces slipping, for a wihle. Additionally, the DMF may also be part of the problem as it may also push the clutch and PP diaphram back on the SC bearing as the DMF starts to fail and seperate.


Like many others here, I had a hard pedal issue after sustained highway driving from the day I bought the truck. Totally empty, very light load, very little throttle application and thus torque. So it wasn't a slipping condition. It had to do with how many miles I drove without using the clutch, not how hard I was working the truck.

Later when I put the Edge box on, it still worked the same for quite awhile. Then I started getting some hard pedal after a high gear romp on the higher Edge levels. Soon after that I was able to make the pedal stick near the floor when I did that. And not long after that, the Edge box became almost unusable, I would make the pedal hard and/or make it stick to the floor any time I used the Edge and put my foot into it. So yeah, this was probably a slipping/heat condition. Although I never detected any slip, in terms of seeing the tach go up faster than it should, relative to the speed.

So after that happened, I got an SBC Kevlar unit (not the dual friction ceramic/kevlar version). This was just after the latest flywheel for the Duramax came out. The SBC clutch came with a new stock GM pressure plate and I got the dealer to put it all in for nothing when they put in the DMF per the service bulletin. Nothing else was changed.

Well, all my hard pedal problems went away completely! I could drive as far as I wanted down the highway, no hard pedal. Early in the clutch's life I could also slip it BADLY by cranking up the Juice in the higher gears, and it never once stuck to the floor or got hard. By badly I mean put your foot down and watch the tach spin up without the truck accelerating.

(at the time, I didn't realize that a kevlar clutch needs a break-in before it gives max holding power. I only did that to it a couple times before it started holding)

So anyway, the fact that I'm using a stock GM pressure plate and flywheel, and the only non-stock piece is the disc, and the hard pedal issue is gone, suggests to me that it's got something to do with the disc.

I am very glad to hear that some of you have pedal problems gone!! I don't think I will ever forget that problem.

ZFMax, That Kevlar plate may be insulating the SC from some heat. I would guess that a Kevlar slipping might not get as hot as other discs that slip? My current dual disc kit, now with replaced parts, does not slip. Changing some parts recently has really reduced pedal pressure and increased pre-load. It works much better but still squeaks and grabs, but less. Anyhow, previously, the kit did have a serious slip (posted on another thread back in Jan 05). However the pedal did not get hard or stuck as you described (I had the exact EDGE impact you noted but I could see the slip on the tach - both on stock and dual disc). Anyhow, with all the great posts here, the experience I have had, and from Peter the fact that the stock PP is self adjusting for pre-load, makes me believe that the PP is probably the largest hard pedal culprit. We may learn something more as you put many miles on your new clutch.


Cheers.

My SBC Kevlar disc has been in there for about a year now, with the latest flywheel.

As I said, when I first got it, there were some slipping issues. Got the motor to spin up without the truck accelerating a few times when I put my foot into it on Juice level 5. Also, while towing a trailer up a 6% grade last October (between SLC and Park City) I couldn't use even level 1, I was getting a hell of a noise if I did. I was, at that point, wondering if I hadn't made a mistake in getting the SBC disc.

The slipping went away and the clutch started holding though, as the clutch broke in. And just a couple weeks ago I pulled the same trailer up that same hill and I could put as much juice to it as I wanted and it worked perfect. The only limitation I had while climbing the hill was the EGT's. Well, and the traffic. I could've gone up in at 80mph no problem if there weren't so many cars in the way.

I did, however, get some hard pedal on this trip, after long stretches with no shifting. That's the first I've seen of the hard pedal since getting the SBC disc. Not bad though.

Overall, I'm real happy with the SBC disc and the new flywheel seems to be holding up, too. I read the service bulletin on this new flywheel when I got the thing and they described the failure mode of the old one. I don't remember all the details but it sure didn;'t sound like something that was power related to me. And my first two flywheels went out at about 30K each even though only one of them ever saw any Juice. So I'm skeptical of all these folks saying DMF failures are related to power.

One thing I've noticed while towing with the ZF is that if I hit the hill at about 80, nothing will slow me down. But if I have to hit it at 70, I'm much more likely to not be able to pull it in 6th and be forced to downshift. The jump from 5th to 6th is too big and I'll have to choose between winding it out in 5th or lugging it in 6th. But if I hit the hill at 80 or better in 6th, it's no problem, it'll go right up the hill with no downshift. The thing has got the power, so long as you're not lugging it.

I have a 01 6-speed, had the same hard pedal, and pedal sticking down problem. Called Peter at South Bend Clutch, put in a button clutch and I'm only 435 HP to the rear wheels but it holds fine and I no longer have the pedal problem.

Based on information I have heard and things I have seen myself, slippage is normal and constant, at minimal levels. A very strong clutch system will slip so little you will not be able to notice it. For instance, traveling along the hwy at some speed watch the engine rpms (a digital gauge like an Edge Attitude), they move up and down constantly (maybe only + or- 20 rpms). So the power to the clutch; which is tied to constant truck weight, tranny gear, road resistance, etc; is going to have a heat impact on the plate.

I have never disassembled a stock PP but looking from the outside it appears that the heat would change the position of the diaphram. Maybe this moves it closer to the SC and then transfers heat?

Regardless, with a hard pedal it must be due to hydraulics pushing back on the MC and pedal. How can it do this? Either extreme heat expanding the fuid and pushing the check valve or the diapharam fingers pushing back on the slave cyl due to heat transfer from the CP to PP? The angle of the diaphram fingers has a huge impact on the pedal hardness and how much pressure is needed to release the clutch. This I know from personal experience.

So my guess is that with a DMF if you can minimize slippage the hard pedal will disappear. But the stock CP will not hold and the DMF, due to many internal parts, will not hold to long term reliability (reliability analysis is partially based on number of parts). Eventually, as ZFMax is starting to see, reliability is a result of all the components and how well they mate up.

Based on all my examinations the whole stock design is flawed. DMF, weak/thin CP, self-adjusting pre-load PP have all failed and been replaced, and failed again.

Anyhow, you all can consider this some chew: :think:

wish you all good luck::Handshake