Dahlenk

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?

battmann

iTrader: (9)

Hawk's motorsports has some:
Disc rear
Drum rear

you could also try ebay.

Dahlenk

Thanks! That helps me out greatly!

QwkTrip

iTrader: (22)

It is obsolete, that's why you can't find a replacement.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

Dahlenk

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

QwkTrip

iTrader: (22)

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.

SbFormula

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

sofakingdom

iTrader: (1)

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.

QwkTrip

iTrader: (22)

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.

sofakingdom

iTrader: (1)

#2 sounds more likely to me...

Dahlenk

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.

Dahlenk

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.

QwkTrip

iTrader: (22)

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

Dahlenk

😆 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. 🤣

Dahlenk

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

QwkTrip

iTrader: (22)

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.