I was having late/hard shifting with my Turbo 400 trans, was told by a mechanic to put a new vacuum pump in, I installed a new vac pump (gear driven) it produces good vacuum to the IP (21 Hg in & 8 Hg out) I also checked at the trans modulator - also 8 Hg, but the truck shifts just as late and as hard as it did before I put the new pump in. Do I need a new modulator?? How can I test it? There are no electronics on this tranny. My truck is an '82 K20 with a 6.5 motor / 6.2 top end, turbo 400 trans, 208 transfer.

the problem is the vacuum switch on the side of the injection pump try to loosen the two ataching bolts and move the switch back and forward with vacuum guage installed at modulator and try to get vacuum up to 15-18 hg if you can't. replace switch and then adjust to this then you can fine tune it less vacuum later harder shifts more vacuum earlier softer shifts

Thanks for the tip 69camarox, I'll see if it works for me. What if I went direct to the modulator from the IP? I am getting 21hg there. Would it build pressure in the tranny?

High vacuum to the modulator can lead to slipping/burning the clutches. Not good!

The higher the vacuum at the modulator, the less pressure to the clutches and the trans will upshift at a lower speed. No vacuum or very low vacuum readings at the modulator will raise the pressures and the upshift speeds. This is why, when the modulator vacuum line falls or is pulled off inadvertently, the trans acts like it won't upshift and you must force it by exceeding the normal upshift speed. When it shifts, it hits hard with a "bang" that can jar your teeth!

This corresponds to a gas engine with an engine vacuum signal to the modulator. At closed throttle, idle or light throttle settings, the vacuum reading will be high. At wide open throttle, vacuum will be low. Hope this helps.

fred is correct you need the switch on the side of the injection pump what this does is simulate engine vacuum of a gasser eg idle hi vac and decreases as throttle opens . vac pump to modulator will give very soft shifts too early with no kick down and no line pressure rise as engine load increases = burnt tranny

Rim, I'm a little concerned about your reading of 21" hg on the inlet side. Is this reading taken at the 3/8 hose bib coming off the vacuum pump or somewhere down the line? Dcam pointed out altitude affects vacuum in a previous post. How high up are you located? The reason I ask is because if you were losing vacuum before you get to the VRV (vacuum switch on the IP) it can affect the modulator reading.
The VRV is a "controlled vacuum leak" as 69camarox points out. In order to have enough vacuum to operate the modulator in the transmission, the VRV has to start out at near maximum vacuum on the inlet side, no bleed-off or leaks prior to the VRV.
Check vacuum directly at the pump and then check it at the inlet hose to the VRV. See if the readings are nearly the same. This ensures a good vacuum source to the VRV. Take a reading on outlet (to the modulator) side and adjust the switch to achieve the highest reading you can get. Most of the time I end up with the VRV pulled all the way to the end of the adjustment slot.
If you can't get more than about 10" hg with the VRV adjusted all the way to the end of the slot, it probably will cause a harsh upshift. There is one more trick that can help. There are adjustable modulators available from NAPA to compensate for low vacuum situations. These are identifiable by looking in the end of the vacuum hose bib of the modulator. If you see a screwdriver slot in the end of the hose fitting, (usually looks like a small setscrew with a hole in the center to allow vacuum to pass through it) you have an adjustable modulator. Back the screw out to soften the shift feel. Be careful, if it comes out beyond the end of the fitting, it will fall out! If you arrive at a normal shift by adjusting the screw. put a drop of Loctite Threadlocker on the threads to hold it in place. Don't get it in the little hole in the adjusting screw as this will block the vacuum flow. Let us know if it works, Fred

Dcam and Fred you guys are right on the money! I have 21Hg at the vac pump, 21 Hg at the VRV, by maxing the adjust slot I am getting 14Hg out to the modulator which just happens to be adjustable. It's like driving a new truck! Firm shifts, good kickdown, beats the hell out of stuffing it to get it to upshift.

One more question if you don't mind, I also have the cruise control servo, would I run a vac line direct from the pump to the servo?
I really appreciate the help guys. :)

Yes, provide full vacuum signal to the small hose bib on the cruise servo. The large line goes to the brake release switch (If your servo has a large line, some do, some don't).