Hey everyone. We are now the proud owners of my father-in-law's 1986 Iroc that he purchased new. The brakes are spongy, and I took it to our local shop. They thought the master cylinder was weak, so we replaced it. This did not fix the issue and also caused the idiot "BRAKE" like to come on. I think it's the proportioning valve because it feels to me that I'm not getting much for rear brakes, and I think that is what the idiot light is plugged into. I have been on the search for a proportioning valve and cannot find the correct one. The lines have bubble-style flares; I only found one that takes standard flares. The parts and service man at the shop has searched without luck as well. Is this a rare unicorn I am searching for, or am I just looking in the wrong places? We hope to take a long summer trip in the car this year, but I want it to be safe. Is there anyone out there that could help me out?

 

Hawk's motorsports has some:
Disc rear
Drum rear

you could also try ebay.

 

Thanks! That helps me out greatly!

 

It is obsolete, that's why you can't find a replacement.

When I questioned Hawks about o-ring material specs and compression they had no clue how to answer. They're just looking at a bin of O-rings and choosing something that they can fit in there.

I have spoken to a specialist who actually understands those things and he did not have correct o-ring.

 

It looks like Hawks only has a coarse thread version that says it fits 89-92. Does that mean mine takes a fine thread?

 

List of prop valves (combination valves) with the broadcast code is in post #4, https://www.thirdgen.org/forums/brak...ng-torque.html
The broadcast code is two letters stamped on the housing that you can see. You have either NC, ND, NB.

Your '86 would have had what is described as M1.0 tube nut thread.
Later on the design changed to M1.5 tube nut thread.

I don't know why they use that terminology, it is completely incorrect. Is it an M10-1.0 thread, or M10-1.5 thread. Both were M10 diameter but the thread pitch changed in later years from 1.0 to 1.5.

 

I like the "They thought" but in fact they did not know and just threw a part at it! If they did not bench bleed the new master cylinder thoroughly, the "BRAKE" light will come on. That indicates air in system. Does the "BRAKE" light stays on all the time or it just comes on when you apply the brake? Uneven pressure from "front and back" will get the proportioning valve to illuminate the "BRAKE" light.

Just saying... peace

 

Prop valves for these cars have always been a PITA.

82-88 discs needed a prop valve specific to the Delco-Moraine cast iron calipers. 89-92 had the PBRs in the rear which are VERY different, hydraulically.

Of course, sometime in mid 84, they went to the metric thread; so, 85-88 would be the year range you'd be looking for, to be safe. I wasn't aware there were 2 different metric threads, butt somehow, doesn't surprise me too much.

The "BRAKE" light will come on if the parking brake is partly set. Check that first real quick. Next, it'll come on if the pressure in the front brakes, is too far different from the back ones. A plunger thing in the combo valve (aka prop valve) is responsible. You can unplug the wire to it, and if the light goes out, then you know it's been set by the pressure differential. It's "supposed to" reset itself if you hit the brakes, and enough pressure is applied, and both fronts and rears pressure up properly. It takes more or less a panic stop to make that happen, like a death ohshit stomp. There's some kinda way to take it apart and reset it butt I don't know what that is. I've never had to do that.

Sb is completely right about "bench bleed". If that is not done, there is NO WAY for the air that's left in the MC, to EVER get out. As I've posted repeatedly, since the air pockets are ABOVE the fittings in the MC for the lines, NO AMOUNT of bleeding will EVER force the air out. You can run 10 gallons of fluid through it EVERY SINGLE DAY by whatever method you want - gravity, vacuum, pressure, MightyVac, WHATEVER - and 20 gallons every Sunday just for good measure, and that air will STILL stay up there where you can't push it out. Only way to clear it is "bench bleeding".

You might want to search this Brake forum about "grainger spring" for further info.

 

It was a mid-year change, so two ideas come to mind....

1. They had a brilliant reason for the design change. In fact it was so brilliant that they ran the change immediately without waiting for the next annual roll up.

2. Someone in Purchasing bought a truck load of factory rejects at a discount because the parts were tapped with the wrong thread.

 

#2 sounds more likely to me...

 

Thanks to everyone. This is all great information. I'm going first to diagnose the idiot light. I have already messed with the parking brake and am certain that is not the issue. The light stays on all of the time. I will unplug the light from the combo valve and see what happens. I will also try to get the valve to "reset" as stated. The brake pedal is spongy. You have to push it pretty far before you notice anything. That is why I first took it to the shop, and they replaced the master cylinder. The light didn't come on until after the new master cylinder was installed, so honestly, all of the information posted so far is very intriguing and quite possible. I'm an amateur at best and have never messed with anything dealing with hydraulic brakes. I will report back in when I have checked these few things out. Thanks again for all of the information and advice.

 

Alright. A few quick results.

The light goes off when I unplug it from the combo valve.

I also did the "panic punch the brakes" maneuver to see if I could reset the valve. That showed me how terrible the brakes are right now. I did it at about 40mph, and I swear it took a 1/8 of a mile to stop!! With no good results.

 

Next time you can just stab the brakes while parked in your driveway....

 

😆 Yeah. I know. But I was curious because I had not tested them beyond dealing with the spongy pedal. We've got some great testing grounds out here. 🤣

 

Would it make sense for me to just find someone in my area that could help me rebuild the combo valve?

 

The GM rebuild kit is long obsolete, nobody has one to do a rebuild.

If you remove the electrical plug from the combination valve, then I think you can manually move the safety switch on center again. My guess is you will find a video about it on YouTube.

 

Curious...what O-ring are you talking about?

 

There are o-ring seals inside the combination valve that if they leak.... you get to enjoy a carnival ride trying to bring your car to a stop.

The seals swell after exposure to brake fluid so matching up sizes is not possible. You have to know what is the dimensions of the surfaces you're sealing to, how much compression should be present after assembly, and what material specs are required for the application. And you have to get it together without the slightest nick on the seal.

This is one of those situations where the less you know the less you're worried about it but the more dangerous you are.

 

ahhh thx! now i know / am worried BUT im less dangerous!

 

The light comes on because there is an imbalance in the brake system hydraulics. This is probably the cause of your spongy pedal and there is nothing wrong with the valve and probably was nothing wrong with the MC. It's hydraulics, isolate with plugs and find where the problem is.

 

Ok - it's been a while, but I finally have an update.

I got the little tool that goes into the combination valve to properly bleed the brakes. This morning, I bled the brakes. There was air in the front lines but none in the back. The idiot light went out, which was great, but the pedal went to the floor, and then the light came back on. Now, the light goes on only when you apply the brakes.

Does this mean the master cylinder/combination valve was not properly bench-bled or do I go through the bleeding process again?

What should be my next step? I've never bench-bled an MC before, especially one with a combination valve. Thoughts? Suggestions? Directions? Any and all would be great. The good news in all of this is that I'm learning a lot. Thanks for all of your help everyone.

 

The light comes on because of an imbalance in the hydraulic pressure between the front and the back brakes. You have something wrong hydraulically in one of those circuits and your combo valve is working properly. Is there any leaks at all? Have you visulally checked every inch of the brake system? You can check the MC by buying plugs for the MC, install the plugs and you will KNOW if the MC is bad or if it needs bleeding. With the plugs in the pedal should be rock hard.

 

I have no leaks and the MC is brand new (which I know doesn't guarantee that it is great.) I'm now just concerned they didn't properly bench-bleed it since that is what caused this light to come on in the first place.

 

Probably.

EVERY brake system these days, and as far back as the 60s, has a combo valve. They first appeared in disc/drum systems. Nowadays they're all that way. Nothing unique or different about it.

Try youtube. Doesn't matter what car's MC they show; it's all the same.

 

No magic to base brake systems or bleeding a MC, it is basic hydraulics. You cannot compress a liquid but you can compress air. It can be done on the car if you want but it's easier on the bench in a vise before installed. I have an assortment of steel lines I cut off of cars at the junk yard for this purpose and a kit with a lot of adapters but the cheapo kit with the plastic fittings will work. All you do is find the right fitting and put lines on that loop back into the reservoir but stay below the fluid level so you don't suck air back. You fill the MC and then pump full strokes slowly and push the air out through the lines. Clear plastic lines let you see the air bubbles, when there are no bubbles you are done.

 

Can't bleed the MC in these cars with it installed. Well, ... maybe. You'd have to park it nose-down on a hill at about a 45° grade. If you can do that, you deserve to make a liar out of me.

So, for all practical purposes, can't be done. Gotta take it off and put it on the bench.

"Suck the air back" is the least of the problems. Getting the line exit ABOVE the air pocket so the trapped air bubble can reach it when it seeks the top, as it inevitably does (in this miserable universe anyway), ... ESSENTIAL. Yes, managing the "suck back" obstacle effectively, increases your odds of quick and more predictable success; I'm NOT saying it isn't A Good Idea; butt it ISN'T "The Problem". The air bubble trapped at the top of the MC and being ABOVE the line fitting, IS.

 

To add to your list of issues, I'd almost bet the rear calipers are stuck along with a crappy new master cyl that probably didn't need to be replaced.

 

"This did not fix the issue and also caused the idiot "BRAKE" like to come on." if the light was out before, then it sounds like something the SHOP did is responsible for your issues.... so likely they didn't bleed it properly since they only touched the master..



I HEARD that the main difference between Disc and Drum P valves is the spring rate, and that threads changed mid 89.... You could maybe do the GRAINGER spring upgrade if you have discs since you have a 86 P valve. which should be the aluminum valve...

there's 3 types of valves that I am aware of design wise...


Early 3rd gen cast iron?

1984-1992 Camaro Firebird Brake Booster Master Proportioning Valve Disc/Drum | eBay


Mid? third gen "stepped" aluminum

1984 Trans Am Brake Proportioning Valve GM OEM | eBay


Mid to late third gen but with 2 different threads
86-89.5 Camaro Firebird Trans Am Brake Proportioning Prop Valve Disc / Disc OEM | eBay





What rear end do you have in the car? If its a10 bolt.... maybe you should go with some LS1 brakes

 

It's a little more complicated than the spring. The shape of the piston also varies, resulting in different proportioning rates. The spring only effects where the hysteresis point occurs (when the valve begins to proportion line pressure different front and rear). You can read more about it here,

https://www.thirdgen.org/forums/brak...ng-torque.html

There are at least 3 different pistons that I know of, maybe more. The same spring used with different pistons can result in big differences in rear line pressure, especially the harder you push the pedal.

This is why when people install the King Kong of springs some will say they hardly notice a difference, and others will dang near crash their cars in panic when the rears lock up unexpectedly.

 

Ok. Another update. Tonight I spent some time with my Dad once again bleeding the brakes. I was able to get a tiny bit of air out of the lines. After bleeding, we had a good, strong pedal. But then I start the car, and the pedal basically disappears, and the idiot light comes on when the brake is pressed. If I pump the pedal a few times, the light stops coming on. The light returns if I wait a few seconds and press the pedal again. This entire time the pedal is rather weak. I then bleed the brakes again, and the entire story repeats itself. I did this four times tonight and then gave up. What the heck? I don't understand how I can have good pressure until I start the car.

 

The power brake booster is designed to boost your foot pressure, the system is failing when the higher pressure is exerted on the MC. You have a base hydraulic problem. You can bleed or you can diagnose. Start at the MC with the plugs I suggested. Keep in mind you are working on a car with parts that are OLD and have been sitting and the brake system on that car sucked when they were new from the factory. Worst rear brake calipers ever designed.

 

this is good to know!

damn youtubers just sayin anything

 

So you bled it and found air each time?


might want to try what beeman / sofa said and just take it off and bench bleed then put on the car and bleed out all 4 wheels.

 

Some folks have said that if you have a combination valve, that it also has to be a part of the bench bleed. What are your thoughts on that?

 

I've heard of this, but personally don't know anybody who has done it. its all bench bleed the master then bleed the rest on the car

 

Was the master cyl OFF THE CAR, and HELD HORIZONTAL, so that the line fittings were higher than the air bubbles inside it?

​​​​​​​If not, you accomplished exactly NOTHING.

 

It would be okay with me if our handsome moderator deleted the 20 year old sticky about Attention: Please Use SEARCH feature .... and replaced it with significant and useful tech.

 

Never heard of that, don't see any reason for it. No magic in that hunk of brass, it's all mechanical and the air will push through it. If you're wanting to you can crack the lines on it and bleed there when you're bleeding.
Did you test the MC? Did you bleed the MC?

 

Can you share some more pics or info on these master cylinder plugs? And does anyone have suggestions on good MC brands?

 

https://www.thirdgen.org/forums/brak...r-upgrade.html Here's fitting sizes now find plugs, do the google or go to a real parts store

 

The soft pedal issue is most likely the crap rear discs used prior to 89, I replaced everything on my system when I worked for a GM dealer back in the early 90's, I went from Drum to 88 rear disc set up and could never get a pedal and yes I used the correct parts from front to back, I finally got a perfect pedal when I swapped on the PBR set up.