1993 GMT400 4WAL - Where is the residual valve?

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Yeti_Owner

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I've done about as much searching as I can on this topic, but not found a definitive answer. Where is the residual valve for a 93 Yukon with the 4WAL ABS system? Is it in the master cylinder, the ABS unit, or in the proportioning valve? Is there any evidence to support one of these locations?
 

96-2D-Hoe

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There is a manual for 93 on this page. May help
67-72chevytrucks.com/vboard/showthread.php?t=700011
 

Yeti_Owner

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Well hmm...there isn't anything in the proportioning valve providing a residual pressure function for the back drums. The "Quick-take-up" function in the brake master cylinder takes care of the low-drag calipers on the front. I didn't see anything in the descriptions for the 4WAL system to keep the drums close. So what does it in this truck?
 

96-2D-Hoe

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Well hmm...there isn't anything in the proportioning valve providing a residual pressure function for the back drums. The "Quick-take-up" function in the brake master cylinder takes care of the low-drag calipers on the front. I didn't see anything in the descriptions for the 4WAL system to keep the drums close. So what does it in this truck?
On my 96 it's just the auto adjustment afaik
 

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The proportioning valve has a sliding piston in it that also acts as a safety in the event of a brake line failure. They can get some wear because the pistons will slide to the side that develops pressure last. Never heard it referred to as a residual pressure valve so I'm not really sure if this is what you're looking for or the reason for the lack of responses.
 
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Yeti_Owner

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On my 96 it's just the auto adjustment afaik

From what I've found in searches, the ABS unit on the 96's handles the proportioning function, unsure of any residual pressure to keep the shoes close to the drum.

Hipster - on pre-abs trucks, the master cyl had a little check valve in the port to hold residual pressure to keep 10psi in the rear brake lines. As I understand it, this prevented the shoes from being pulled all the way back by the springs, lessening the amount of fluid and time needed for the rear brakes to work. I'm trying to figure out what, if any, device provides that function for a 93 4x4 with the 4WAL ABS system.
 

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If it helps, I'm replacing wheel cylinders ( and recently replaced rear brake pipes) on a 1999 C2500 with ABS and there's no pressure in the line when slackening the fittings and if the shoes were any closer to the drums the brakes would be permanently on and on fire.

The self adjusters seem to work well.
 

delta_p

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The quick take up function serves both the front and rear brakes because the volume of fluid delivered across low pressure MC chamber is enough to drive the secondary piston as well as the take up for the primary circuit.

If you go to this link and download the "Base Brake Systems" it'll walk you through all the functions.
https://www.gmt400.com/threads/88-95-service-manuals.43575/
 

Yeti_Owner

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The quick take up function serves both the front and rear brakes because the volume of fluid delivered across low pressure MC chamber is enough to drive the secondary piston as well as the take up for the primary circuit.

If you go to this link and download the "Base Brake Systems" it'll walk you through all the functions.
https://www.gmt400.com/threads/88-95-service-manuals.43575/


I guess - there is a ton of conflicting information on the forums. Folks are putting random master cylinders on their trucks, residual valves, all kinds of nonsense.

From the service manual for a 94 it says this:

"The master cylinder (figure 1) is designed for a system using low-drag calipers. In addition to the standard master cylinder functions, a quick take-up feature is included on models with a vacuum booster. This provides a large volume of low pressure fluid to the wheels with the initial brake application. The large volume of fluid helps overcome the clearance created by the caliper pistons and rear brake shoes retracting."

I guess that's why the pedal sucks on these, huh? The big "squish" before much happens is because of the QTU function.

My issue is I did an ABS delete a year ago (the master, both front brake hoses and one rear brake piston were replaced at the same time), and if I really mash the pedal, I get a brake warning light. I've had it bled multiple times... As far as I can tell, the brakes themselves work fine, but the pedal is still mushy and it triggers it. What's throwing that combo valve out to the point it triggers that light?
 
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