[Joe37]

I am looking for information on just how the combination valve / proportioning valve works on '79 with four wheel disk. Bleeding the system is proving harder then it needs to be. The brass shuttle valve needed to be moved just to get fluid to the rear line. Does anyone have a shop manual that might show its operation?
Joe

[ta6point6]

I am not sure what you are asking but the proportioning valve works like all brake prop valves directs brake fluid to the calipers. That being said these early 4 wheel disc brake set up on 79-81 TAs were not the easiest to work with. Have you recently rebuilt your calipers and need to bleed? The rear calipers are the tricky part usually and needed to be set up correctly with emergency brake cables. Is your brake light on and got tripped on proportioning valve?

[Joe37]

The whole system is new except the proportioning valve. I pulled it apart, cleaned, and replaced the o-rings. I still wasn't getting fluid out of the rear line so pulled apart again. I found the rear portion with the heavy spring was still rusted together, first time I thought it one piece. I guess what I'm asking for is a fluid path through the thing. The fronts are easy enough to see how it goes, the rear portion seems like the spring isn't really doing anything.

I have the calipers adjusted with parking cables on. I have fluid at the back, but with the bleeder open, I barely get any fluid out, and the pedal is super stiff. Master cylinder is smooth and works easy, and is bleed out.
I will reinstall the proportioning valve now that the spring and valve are free to move and see what happens. Pretty sure I can't find a rebuild kit for the valve, so a new one may be coming. If I can get fluid through it, pretty sure the brakes will work like they are suppose to.

[Joe37]

I found this online, and parts from Muscle Car Research, so hopefully I can save it.

[TheSilverBuick]

I haven't tried it yet, but I recently bought one of these to thread into the "Switch" port to keep the valve centered during bleeding.

[78w72]

As mentioned above, the valve needs to be centered to get fluid to all corners. The tool above works but there is a procedure in service manuals on how to center it using the pedal but I forget the exact details, something like cracking open one caliper bleeder then stomping on the pedal, I had to do that many years ago with a mechanic friend helping.

I just installed new lines from the master to prop valve that had one start to leak & rear calipers on my 81 TA, luckily the prop valve was in working order so no issues bleediing & the breaks work great... parking break adjustment can be tricky but is pretty simple once you understand hos it works.

If you cant get the original to work right, the aftermarket brass prop valves are nice & work good.

[Joe37]

Both pictures / drawings I have found show the valve design as fitting disk / drum setups. Maybe they used the same valve for disk / disk, I don't know. I am starting to think the spring and cup seal proportioning section is to hold pressure on the rear wheel cylinders in drum brakes. I can't see how it would do anything else. I also don't see how it would help a rear disk brake caliper.

[Joe37]

Getting deeper in to this project, I asked over on the HAMB site, there are a lot of very smart members over there that work on a wide range of stuff.

The proportioning part of the valve is to hold off pressure to the rear brakes for just a bit, nothing drastic, just enough for the fronts to start working.

I'm still not 100% sure how it does it since the fluid and the spring are both going the same direction. Best I can come up with is the chamber with the valve and spring pressurizes then is forced open by the cup seal, opening the port to the rear line.

Proportioning valve are vehicle specific due to weight, brake type, pressure, and car type.

[78w72]

Have you tried to center the plunger yet to get yours working right? There should be info out there on how to do that.

[Joe37]

Heres where it is now, waiting on new seals. Pretty sure the proportioning valve was holding up the fluid, it was rusted in the hex nut not letting it open enough. I will know once I get new o-rings and seal in place and back on the car.

[Skip Fix]

I just put a T in the front and an adjustable in the rear line.

[Randy_Handel]

I have a 1980 Trans Am Pace Car that has 4 wheel disc brakes that I'm having some issues with grabbing/dragging after driving it for a while. When it first started happening, I found that the master cylinder was leaking so assumed that was the issue. Replaced that and it is still happening. Both front and rear calipers are original, I took everything apart and cleaned everything up, so I don't think they are the issue.


I have ordered the rear single and both front rubber brake lines that I'll replace next but found this thread trying to figure out what is going on so I thought I'd post and see if @Joe37 ever figured out his issue?

I have the tool to hold the proportioning valve in the middle now but am wondering if maybe I should rebuild, just clean up or replace mine if the rubber brake lines are not the issue.

[Joe37]

The one I rebuilt is on the car and working now. It's still in the paint process so no drive time. I found the portion going to the rear was rusted in place not letting any fluid through, top of the picture, the silver shaft with lip seal was stuck. After the cleaning and new seals it was still hard to bleed the system. I had a hold off tool, but it didn't help much. What I did was fabricate a 1/4" thick clear plastic lid with a threaded hole to fit over the master cylinder with a cork gasket. I had a fitting for a vacuum line threaded into the hole. Using my truck as a vacuum source with a long hose, I put the end of the hose up to the fitting but not over it. The vacuum was enough to pull some air out of the rear system, I did it a several times and each time it would pull more air. Too much vacuum will pull air in through the caliper seals, so I was careful with the hose. The vacuum worked so well because the proportioning valve is designed to not let fluid to the rear brakes till the fronts have started to work, by pulling the fluid backwards, there was no restriction and the air came right out. Another '79 owner called about his brakes, so I explained how I bleed them. He did his the same way and in just a few minutes had a solid pedal using the vacuum method.

I don't believe the valve could cause a random brake drag. If both rears or both fronts are dragging, then maybe it could. But if it's just one side, then no.

You need to check for a hot rotor after driving to see if one or more is causing the issue. All new rubber hose is a good idea if they are original or fairly old. A brake booster could also cause a problem if it doesn't return all the way. The push needs some clearance or the master will not return fully which can block the ports and hold the brakes on. Go for the hoses, I've seen them burn up rotors.

Each body style had its own proportioning valve with the correct spring pressure for the car it was designed for. The aftermarket ones fit and work, but may not be 100% correct for your application. Rebuilding them is pretty straight forward once you have the seals. These guys are pretty smart,
https://musclecarresearch.com/kh-kit-1

[Randy_Handel]

Both front and rears are gabbing/dragging. They don't get hot, I've checked. I say grabbing because its like they kind of stutter after you stop and try to go again then they must release.

I replaced both front and rear rubber brake lines and it didn't help.

I replaced the master cylinder and also used washers to change the pushrod to master cylinder distance to the point the pedal travel is too long so I am pretty sure the ports aren't being blocked.

My combination/proportioning valve is one that isn't rebuildable, I guess GM started using them in 1980?

Per ChatGPT (which I'm the first to admit isn't always correct):

Part #: 1257189

Used on 1980 Firebird/Trans Am with 4-wheel disc (WS6) – one-year / very low-volume piece

Manufactured by Delco Moraine

It is a sealed solid-body combination valve

No threaded metering cover and no external proportioning plug like the common disc/drum valves. This style cannot be disassembled from the ends the way typical GM combination valves can. The metering and proportioning pistons are loaded from inside during casting and crimping, and the brass body is staked or swaged closed. That’s why you don’t see removable caps.


So, at this point I'm just going to order a new one, and if the problem continues, I'll replace the calipers. My gut tells me its been the calipers all along but logic says it must be something upstream since both front and rear are doing it, if it was calipers would the front and rears both start acting up at the same time?

[Scarebird]

Those calipers have ALWAYS been problematic, and most are rebuilt incorrectly. Note to adjust the e-brake FIRST before bleeding if you start over with different calipers.

[Skip Fix]

I have also found with those rear calipers silicone DOT 5 fluid helps a lot with the ratchet mechanism

[provocating]

I went through all of this with my 1979. I had the same sealed proportioning valve on mine, could not rebuild it. My brake pedal was going to the floor, one of the rear calipers was stuck even though it looked brand new. I replaced the proportioning valve, replaced that rear caliper and finally after a good bleed I have a full pedal and the brakes stop perfect. Now I routinely pull my emergency brake once a month to keep those rear calipers adjusted.