Combination valve

[someotherguy]

There might be some confusion with terminology. The combination valve has a metering valve on the front and a proportioning valve on the rear. Some trucks appear to have only the metering valve on the front with the rear made up of just solid aluminum, but the body of the valve appears identical.

All trucks have ABS.

All GMT400's do, anyway. I saw someone mentioning squarebodies in the discussion and that's where we get into non-ABS territory.

Richard

 

[67GMC]

It's also strange that there is no mentioning of how to adjust this "proportioning" valve in the service manual I have for my 1988. I'm wondering if the incorrect part is attached to the poster's truck.

I don't think a proportioning valve is needed with ABS systems. From what I have read, on other trucks, the proportioning valve only works during hard braking to prevent the rear wheels from locking up prematurely (allowing more force to the front disk brakes). As the Rear ABS would sense the wheel lock at the rear wheels anyway, the system would try to prevent this from happening using the technology. I would think a mechanical proportioning valve would get in the way of an automatic RWAL system. Just my thoughts.

 

[Gibson]

It's also strange that there is no mentioning of how to adjust this "proportioning" valve in the service manual I have for my 1988.

You don't adjust them, the bias is built in.

I don't think a proportioning valve is needed with ABS systems. From what I have read, on other trucks, the proportioning valve only works during hard braking to prevent the rear wheels from locking up prematurely (allowing more force to the front disk

It's really a "combination" valve,, it combines different functions.
1, It serves as a switch that activates a light to warn of a front or rear brake failure.
2, It has a "hold off" feature that retards pressure to the front brakes until the slack of rear shoes is taken up, and the rears start to work.
3, It limits the rear pressure,, on the trucks, the rear pressure is in the 600psi range for the 1500s and the 2500 Suburbans with the JB7 brakes,, although in the 2500s with over 8600GVW, and the 3500s the range was about 800psi.
The pressures and its operation are not perfect,, it's a passive mechanical device, it doesn't know if the shoes/drums are worn or if the brakes are not in adjustment, and it certainly does not know, or care if the rears lock up.
The combination valve does not and can not prevent rear lock up,, that's the job of the ABS.

 

[someotherguy]

It's also strange that there is no mentioning of how to adjust this "proportioning" valve in the service manual I have for my 1988. I'm wondering if the incorrect part is attached to the poster's truck.

You don't adjust them, the bias is built in.

Exactly. 67GMC, what you're thinking of would be an adjustable proportioning valve which is an aftermarket part for use in racing applications/custom braking systems...not the OEM part being discussed here.

Richard

 

[Schurkey]

If you've got a height-sensing proportioning valve, I bet there's a way to adjust the lever to "calibrate" the valve to the suspension height.

 

[HillcrestFarm]

I checked and my 1994 Blazer has the same valve, with the 4wal. This confirms a previous post around the SUVs having this style valve


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[Steven Petersen]

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I’ve just been reading this. I just installed a NOS unit today on my 88. Combination Valve/Isolation dump valve. All as one unit, which is the only way the 88 parts catalog shows it. You can see in the pics the valve has the proportioning valve on the rear.

 

[Steven Petersen]

I was just looking through my 88 service manual supplement and noticed this about the brake valves. It seems GM designed both valves and connecting pipe to only be replaced as a single unit.

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[Supercharged111]

I guess they don't like to party.