ABS delete / Proportioning valve bleeder tool / 1988 K1500

[WV_Dave]

I used my thumb while my wife pumped the brake pedal. Took a couple times until you get enough fluid on the other side of the prop valve then it stays centered.

It's a safety feature in case you lose all pressure to the rear line. When too much fluid rushes past the valve it pulls it over and closes that path, so you don't lose all your fluid on the ground and still maintain pressure to the side that works. While the brake pedal is being pressed, just push the needle in with your thumb nail. Each time will let a little past until the line is full enough that the pressure doesn't trip the safety valve.

Press the pedal down and hold
Open the bleeder on the passenger side wheel cylinder (nothing will come out)
Go up front and press the needle in
Back to the wheel cylinder and close the bleeder
Release brake pedal

Rinse and repeat until the needle stops tripping. The brake light will go off on the dash.
Then bleed the rest of the air out as normal.

 

[Schurkey]

What "needle" are you talking about?

Photo?

 

[WV_Dave]

On the front of the prop valve, behind the rubber cover. I should have been more clear, my post was a direct response to the one before mine asking how to reset the shuttle valve. I got in a hurry, I meant to quote him.

 

[Schurkey]

Your explanation is voodoo.

That's not the "prop" proportioning valve. The proportioning valve is at the rear of the COMBINATION VALVE. The "needle" you're referring to is the stem of the METERING (holdoff) VALVE. It has NOTHING to do with the safety switch. The metering valve affects the FRONT brakes. Pushing that pin is a recommended procedure if you cannot get fluid flow to the FRONT brakes. it has NOTHING to do with the rear brakes. See Figure 2-12, and the text at 2-23 and 2-24.

You must be registered for see images attach

 

[WV_Dave]

What you posted a picture of is a combo valve. Mine, on the other hand, only has one line in and one line out for the front brakes, so I don't think I have a metering valve in mine. You can see in my picture. Making that unit just a prop valve, right?

I had a brake line rupture, the hard-line on the rear diff going from the splitter to passenger side wheel cylinder. The Brake light came on as if I had my emergency/parking brake set. I replaced everything from the rear rubber line on back so there was a lot of air in the system. I could open the bleeder and no air or fluid would come out no matter how many times you pumped the pedal.

I read, on this forum, what to do. Which I've already described above so I'm not going to again. It worked, I got fluid to the rear and everything bled out great. It took about 4-6 times before it stopped tripping (and by tripping I mean that needle in the front would be pushed out about 1/16th of an inch further than it supposed to be and the Brake light would come back on). My front brakes were never opened during this process until I had the rear completely bled. I tried bleeding the fronts only then just to make sure but no air was in them.

I'm guessing voodoo too. There's apparently more than one type and I'm guessing if you have the combo valve a special tool is needed to release the rear valve, but if it's like mine, then the needle looking thing on the front actually resets the rear? I'm just now becoming aware of this so if my model is different then my explanation isn't going to help in this case.

 

[Schurkey]

What you posted a picture of is a combo valve. Mine, on the other hand, only has one line in and one line out for the front brakes, so I don't think I have a metering valve in mine. You can see in my picture. Making that unit just a prop valve, right?

Of course not.

You must be registered for see images attach

Picture isn't the greatest for this, but here goes:
Two brake tubes on the top side. One is entirely visible, coming from the rear of the master cylinder, and going to the front of the combination valve. The rear of the master cylinder supplies fluid to the FRONT brakes, it's plumbed into the Metering (holdoff) valve section of the Combination valve. The LF and RF brake plumbing splits somewhere out-of-view in this photo.

The second tube on the top side would be coming from the front part of the master cylinder, although that's not visible in this photo. The front part of the master cylinder provides fluid pressure to the rear brakes. It goes to the REAR of the combination valve, which is where the proportioning valve is located.

In between those two tubes on the top side, is the Safety Switch.

Metering valve + Safety switch + Proportioning valve = Combination valve.

I had a brake line rupture, the hard-line on the rear diff going from the splitter to passenger side wheel cylinder. The Brake light came on as if I had my emergency/parking brake set. I replaced everything from the rear rubber line on back so there was a lot of air in the system. I could open the bleeder and no air or fluid would come out no matter how many times you pumped the pedal.

So the brake pedal was high and firm when you pushed it? Or did it go nearly to the floor before firming-up?

I'm guessing voodoo too. There's apparently more than one type and I'm guessing if you have the combo valve a special tool is needed to release the rear valve, but if it's like mine, then the needle looking thing on the front actually resets the rear? I'm just now becoming aware of this so if my model is different then my explanation isn't going to help in this case.

I have NEVER needed to dick with the metering valve stem in order to bleed brakes in 45 years of working on cars/light trucks.

I have had to screw with safety switches TWICE in that time--both times on Fords that don't self-center like most (all?) GMs.

 

[WV_Dave]

All you did was explain how the plumbing works from the MC to the splitters. Explain how you can have a metering section of a combo valve if it doesn't also differentiate the two front brake lines at that point. It's seems obvious to me what the metering section does in the pic you posted and how it couldn't be in a unit like mine.

Yes the brake pedal was firm and as high as these trucks get with no pressure being sent to the rear to bleed off air/fluid until the valve was manually centered.

It's happened to many people, not just myself, hence all the posts.

 

[Schurkey]

All you did was explain how the plumbing works from the MC to the splitters. Explain how you can have a metering section of a combo valve if it doesn't also differentiate the two front brake lines at that point. It's seems obvious to me what the metering section does in the pic you posted and how it couldn't be in a unit like mine.

The illustration is old enough to be before the GMT400s were designed. That combo valve also included the distribution block for LF and RF brakes.

Splitting the LF from the RF can happen anywhere in the plumbing downstream from the actual metering valve device. In the illustration, it's spit in the same brass housing as the metering valve instantly downstream of the metering valve seal where it contacts the stem. In the GMT400 it's split farther downstream. This has ZERO effect on the metering valve which affects BOTH front brakes, not LF and RF separately.

You pushed the stem of the metering valve. Maybe some movement of the unit from your finger pressure, or maybe just total coincidence, jiggled the safety switch bobbin to the center position. It should be spring-loaded to the center position anyway, based on where the safety switch itself contacts the tapered sides of the bobbin that pushes to one side or the other due to hydraulic pressure imbalance.

Pushing the metering valve stem is not designed to center the bobbin. What you did "worked" but WHY it worked is still in question 'cause that's not a design feature.

 

[Illbedipped]

Where is this forgotten bleeder on the ABS? I've looked over by 92 C2500 and can't find a bleeder under the hood near any of the RWAL stuff.

 

[Schurkey]

Where is this forgotten bleeder on the ABS? I've looked over by 92 C2500 and can't find a bleeder under the hood near any of the RWAL stuff.

Iron-body RWAL have a single bleeder screw.
Aluminum-body RWAL have no bleeder valve, must use a scan tool to chatter the valves.