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That is not a GM combination proportioning valve. That is a straight-up adjustable proportioning valve. With that, it is possible to adjust the rear brakes such that you can do burnouts while standing on the front brakes, and little to no pressure will get to the rear brakes. That could be your whole problem right there. It could also be adjusted to limit power to the rear brakes to balance the car.
To answer your "Brake" light question, the light is on the center of your dash, and will illuminate red showing the text "BRAKE". You should see it flash when you start the car (there is a ground in the crank circuit to test the bulb), it will light when you pull up on the parking brake handle (normally closed switch grounds the bulb, and opens when the parking brake handle is fully down), and if there is a front-rear brake pressure discrepancy, the stock GM combination valve will lock out one side of the circuit, illuminate the "Brake" light, and hopefully permit the other half of the circuit to stop the car.
Do you have a stock combination valve? It looks like this (borrowed from this site):
Correct booster? IDK. If you go by the color, I would say no. If you go by the size I would say yes.
Somebody sure bubba that Proportioning Valve. It needs to come out of there. It has no safety features. Some call that a bias valve or adjustable Prop Valve. If you order a new Prop Valve make sure its the correct yr. They changed design on that too. A correct unit will run you around $90.
When you get your new MC, before you do anything else, you need to check the booster rod to MC piston gap. There is a tool to check that gap, but its one of those specialty tools that you use it once and not likely again.
Another easy / cheaper way is to go to Walmart toy dept. Buy an "egg" of Silly Putty. This is a very clean clay that is easy to remove. Take a small ball of it and place it on the back of the MC piston. Install the MC on the car. Snug both mounting nuts. Remove MC and carefully remove your clay imprint. That is your gap. I believe it should be 0.060. Too loose and you have too much pedal travel. Too tight and the pads will constantly rub the rotors.
Be very careful about what you insert into the MC line ports. Deep inside is a flare that is cast. If you put a bolt in that port, you will damage the flare.
Click on my profile, look for albums, bench bleeding tips when your new MC arrives.
Also, there should be a felt dust boot / gasket between MC and booster when all is said and done. It will have no effect on your gap measurement.
Last edited by HeadsU.P.; Oct 28, 2020 at 06:37 PM.
Correct booster? IDK. If you go by the color, I would say no. If you go by the size I would say yes.
Somebody sure bubba that Proportioning Valve. It needs to come out of there. It has no safety features. Some call that a bias valve or adjustable Prop Valve. If you order a new Prop Valve make sure its the correct yr. They changed design on that too. A correct unit will run you around $90.
When you get your new MC, before you do anything else, you need to check the booster rod to MC piston gap. There is a tool to check that gap, but its one of those specialty tools that you use it once and not likely again.
Another easy / cheaper way is to go to Walmart toy dept. Buy an "egg" of Silly Putty. This is a very clean clay that is easy to remove. Take a small ball of it and place it on the back of the MC piston. Install the MC on the car. Snug both mounting nuts. Remove MC and carefully remove your clay imprint. That is your gap. I believe it should be 0.060. Too loose and you have too much pedal travel. Too tight and the pads will constantly rub the rotors.
Be very careful about what you insert into the MC line ports. Deep inside is a flare that is cast. If you put a bolt in that port, you will damage the flare.
Click on my profile, look for albums, bench bleeding tips when your new MC arrives.
Also, there should be a felt dust boot / gasket between MC and booster when all is said and done. It will have no effect on your gap measurement.
Gravity bleeding takes a while.....crack a bleeder.....put a hose on it into a mason jar of brake fluid......the fluid will eventually come out and push the air out which will show up as bubbles in your submerged hose. When the bubbles are gone......that caliper is done. I am usually at about 45 minutes per caliper......it doesn't happen immediately, especially on the rears.....
For me it is a 6 pack job.....as I don't drink when I work on things usually and it is a brainless task so I can get shitfaced while claiming to be "working" on something.......I usually have some British Murder show streaming in the background......point is: Don't think about it too hard......hydraulic fluid always finds its way out.....count on that.
Last gravity bleed I did was on a Wilwood disc conversion kit for a 71' Demon with Dakota retro-fit master cylinder, Wilwood adjustable proportioning valve and my own lines......both sides bled out in about 15-20 minutes and I didn't even get to drink 2 beers
Jebby
Jebby, thanks for the info above. I will certainly not touch my pedal. do you think the Motive Powerbleeder would cause a problem to that valve?
Correct booster? IDK. If you go by the color, I would say no. If you go by the size I would say yes.
Somebody sure bubba that Proportioning Valve. It needs to come out of there. It has no safety features. Some call that a bias valve or adjustable Prop Valve. If you order a new Prop Valve make sure its the correct yr. They changed design on that too. A correct unit will run you around $90.
When you get your new MC, before you do anything else, you need to check the booster rod to MC piston gap. There is a tool to check that gap, but its one of those specialty tools that you use it once and not likely again.
Another easy / cheaper way is to go to Walmart toy dept. Buy an "egg" of Silly Putty. This is a very clean clay that is easy to remove. Take a small ball of it and place it on the back of the MC piston. Install the MC on the car. Snug both mounting nuts. Remove MC and carefully remove your clay imprint. That is your gap. I believe it should be 0.060. Too loose and you have too much pedal travel. Too tight and the pads will constantly rub the rotors.
Be very careful about what you insert into the MC line ports. Deep inside is a flare that is cast. If you put a bolt in that port, you will damage the flare.
Click on my profile, look for albums, bench bleeding tips when your new MC arrives.
Also, there should be a felt dust boot / gasket between MC and booster when all is said and done. It will have no effect on your gap measurement.
I will order this proper proportioning valve and try to sort out the piping arrangement. Could you describe what the sensor connection looks like. I am hopeful it’s just tied off somewhere but not sure what I’m looking for.
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
If you get a new master cylinder check that it matches your old one as they have a 'deep' or 'shallow' hole.
Bleeding the M/c. (you know how but here's a refresher.
The rod inside the booster to M/C can be measured with silly putty or you can get this tool.
The brake light in the dash serves two functions, to let you know the parking brake is 'ON' or if the brakes have a problem.
Does the dash light go on with the parking brake applied?
The prop valve Bikespace pictured and new lines from the M/C to prop valve can be ordered from a Corvette sponcer.
That wire harness from the brake warning dash light should have a rubber weather proof boot on it. But like I said earlier, they changed the plug. So not all yrs will fit correctly. And, they went from a steel switch to a plastic one with a different terminal. Fun-fun-fun.
What I am getting at, in the C3 brake parts world is: don't trust the parts counter person to get it right. ***
*** Does NOT fit all C3s.
Last edited by HeadsU.P.; Oct 29, 2020 at 09:56 AM.
