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Discussion Starter · #1 ·
I have a 96 Gmc 6.5 td k3500 4x4. When I bought it, it had a rotted rear brake line. Truck had been sitting as far as I know about 5 years. I replaced: brake lines, rubber brake hoses, wheels cylinders, shoes, calipers, pads. I bleed the system, also manually bleed the abs system after realizing I still had 0 brakes. Got ansy and installed a lift kit on it, went through the whole process again. Same thing. Engine off: pump brake pedal, builds pressure but never hard as a rock. Start engine: goes straight to the floor while applying brakes. Clean fluid. Power steering pump works as it should. Before I replace and rebuild the entire system, can anyone give me some advise and thoughts?
 

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Welcome to DieselPlace
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Possibly a seal in the master cylinder leaking by not allowing the pressure to build up. Did you get the rear shoes adjusted properly. I had a issue with mine that was similar to yours. Didn't have much for brakes till about half way down. Adjusted the rears and now barely step on the petal and almost puts you into the windshield.
 

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Welcome.
How did you manually bleed the ABS?
 

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Discussion Starter · #5 ·
Reading online, I seen that there is a bleed valve on the side closets to the fender. Went through the steps, reblead the brake lines. I know that it's possible that is could still have air in it but I do not want to rule anything out. I also tried the ole take it down a gravel road, stomp on the brakes, take home and bleed but still no change.
 

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Discussion Starter · #6 ·
Possibly a seal in the master cylinder leaking by not allowing the pressure to build up. Did you get the rear shoes adjusted properly. I had a issue with mine that was similar to yours. Didn't have much for brakes till about half way down. Adjusted the rears and now barely step on the petal and almost puts you into the windshield.
If there is a seal leaking, shouldnt I be able to see a leak somewhere?
I havent thought about giving the rear another look over.
 

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Did you replace the master cylinder?
 

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If there is a seal leaking, shouldnt I be able to see a leak somewhere?
I havent thought about giving the rear another look over.
If it is leaking the rear seal would be leaking into the hyrdoboost. Sometimes you can see it sometimes you have to unbolt the master and pull it forward and see if it's wet. There are also seals inside the master that could be bleeding by that are not allowing pressure to build
 

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If it sat that long with old brake fluid in it and rotted lines. The bores in the master cylinder may be corroded and etched, not holding pressure.
 

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Bad Hydroboost? Had same problem on my old 6.2. Would build pressure with engine off, start engine and pedal went straight to the floor.
 
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Does pulling the ABS fuse change anything ?
 

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1995 Chevrolet Silverado Z71 6.5 TD
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Check the weep hole at the back of the M/C to see if there is fluid there. I have had a leaky M/C take out my hydro-boost before.
 
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Discussion Starter · #13 ·
I pulled the master cylinder, the rear seal was dry. Went ahead and replaced it with a brand new one. Blead the system. But still goes straight to the floor after starting. The pedal gets hard with the truck off except on start up.
 

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Discussion Starter · #14 ·
Currently removing the rear drums to see if they need adjustment, will update shortly.
 

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Did you bench bleed the new MC before install?
 

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Discussion Starter · #16 ·
Adjusted the rear shoes. Seemed to do a bit better if you put it in gear. Seemed a little tighter until you started driving then it was the same feel from before.
 

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Discussion Starter · #17 ·
Did you bench bleed the new MC before install?
I mounted it on the hydroboost and poured fluid in it. Let it stand for 5 minutes waited for the bubbles to stop before hooking the lines up. So no I didn't bench bleed it. In the past I've done the same thing with no issues. Honestly didn't know you actually had to bench bleed one. After hooking up the lines, I bleed the abs module. Got a lot of air before it was a full stream, them went to the wheels.
 

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Bleeding the master cylinder must be done before install. Other wise you just inject air into the brake system requiring a complete bleed again. This vid shows both methods. Most new MCcs come with the plugs used in the first method.
I usually use the second method but both work. Auto parts stores carry MC bleed kits with the tubes they are cheap.
 

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1995 Chevrolet Silverado Z71 6.5 TD
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If there is a seal leaking, shouldnt I be able to see a leak somewhere?
I havent thought about giving the rear another look over.
FYI: if the MC rear seal is leaking there is a weep hole between the MC and HB where the two join you would see it there. Either brake or power steering fluid. Smell test
 
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