1993 GMT400 4WAL - Where is the residual valve?

[Pinger]

Couple of questions about these residual valves on GMT400s.
How many on one truck and where are they located?
Do they flow in the direction of the brake freely enough to permit gravity bleeding?

Interesting discussion BTW - cheers.

 

[delta_p]

Couple of questions about these residual valves on GMT400s.
How many on one truck and where are they located?
Do they flow in the direction of the brake freely enough to permit gravity bleeding?

Interesting discussion BTW - cheers.

They're aren't any on GMT400's.

 

[El Tigre]

Unless one installs one to the rear brakes as I did... Yes, fluid flows out freely.

 

[delta_p]

Anyone interested this link shows an old time residual valve like when they used to be part of the master cylinder. I suspect these days with rubber compounds available its just a rubber slug that can compress under pressure to allow the flow to the wheel cylinders.

https://images.app.goo.gl/9k57xz9pCaEHpevQA

Basically the small spring is extremely light and allows flow in the direction to the wheel cylinders with almost no force. But it closes up at no pressure differential or reverse differential. The high pressure differential now going the other way because the pedal force is gone or the fluid heats up and expands, opens the seat of the large spring until the reverse pressure difference is such that the spring closes, ~10 lbs.

 

[Yeti_Owner]

Are you sure there isn't a slight fluid leak or the rear shoes aren't adjusted good. Anything like that may allow pressure to build on one side of the combo valve slightly ahead of the other side on hard braking and shuttle the red brake warning light.

You nailed it. The back brakes were pretty far out of adjustment. So much for "automatic adjusting" rear brakes, huh?

No more warning light...

 

[delta_p]

Excellent! Thanks for posting back on the problem and fix.

Yeah I hardly ever use my emergency brake and I think that is what keeps them adjusted. I usually try make adjustments whenever I have the wheels off and a little time.

 

[Pinger]

Excellent! Thanks for posting back on the problem and fix.

Yeah I hardly ever use my emergency brake and I think that is what keeps them adjusted. I usually try make adjustments whenever I have the wheels off and a little time.

I use the 'parking' brake and mine were (self) adjusted perfectly. I've also read they adjust with the service brakes when reversing - and mine gets that too!

Wonder which it is?

 

[Yeti_Owner]

Excellent! Thanks for posting back on the problem and fix.

Yeah I hardly ever use my emergency brake and I think that is what keeps them adjusted. I usually try make adjustments whenever I have the wheels off and a little time.

I religiously used my e-brake, but it didn't do anything to keep the back drums in adjustment, lol.

 

[Pinger]

I religiously used my e-brake, but it didn't do anything, lol.

Mine self applies - as in if I'm rolling at walking pace and just touch the pedal it rams itself on and the truck just stops. No slight application is possible.

Since I've replaced the wheel cylinders the service brake at very slow speed is just as savage. Is this how it's supposed to be?

 

[HotWheelsBurban]

From what I remember about our 90 R2500 Burb, that's the way it was when the brakes were working. Different series truck but mechanically pretty similar. Was vacuum power booster though; but when working properly that truck would stop well for its size and weight. Hydroboost works even better, that's one reason bigger trucks have it. Our 06 Denali with hydroboost and 4 wheel discs would stop on a dime and give you change LOL.
We did have to put a booster on the 90 at one point. When it was going out, you really had to plan your stops! I want to say we changed one on the 99 but don't really remember. Might have been one of the other square Burbs. After 8 or 9 Burbs it's hard to recall exactly....