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HI all,
I recently rebuilt the rear suspension in my '81. During that process, I replaced the rubber brake hoses and flushed both the front and rear lines of old fluid using my Motive pressure bleeder. I followed the usual rear first, then front's procedure. I noticed after about 50+ miles of driving that the rear rotors show almost zero wear on them (after being resurfaced). Did I stick the proportioning valve to the fronts by chance? I don't show a dash light but then again, I'm not sure it even works. Dang hard to get to for checking. Anyway, looking for advice on the best way to determine the issue and fixes. Thanks!
Sounds like the rear brakes are not doing diddley squat. Is the car "nose diving"?
However, it's been said that the fronts do 80% of the braking anyway.
Just curious, were the rear brake hoses of quality, name brand? There are some real junk units on the market. The price is the give-away.
If the hoses are of low quality, perhaps they have semi collapsed right from the beginning.
Something is blocking flow to the rear calipers. I doubt it's the proportioning valve, they seldom cause problems.
I wonder with all the work you did if some dislodged crud is blocking ports in the caliper? In other words, the four rear bleeder screws flow great when pressurized but the caliper pistons are not expanding in their bores.
And last. Slight possibility the piston seal for the rear reservoir in the Master Cylinder is slacking off.
Well when I bleed them they are definitely flowing fluid. I can see the pads moving ever so slightly when the pedal is depressed. However, they are definitely not biting down on the rotor. Other than that valve, I can’t think of anything else that might cause this? I’m stumped…
I'd crack a bleeder and make sure you get flow with a helper pushing the brake pedal.
The "brake" light comes on in one of three ways, all of them provide a ground. Does it work the other two ways? Parking brake, and while cranking the engine? You can pull the connector, short it to ground, and it should light the light, too (with IGN "ON")
If everything is working properly, you should be able to lock up all 4 wheels in a panic stop.
One last thing, then I'm stumped also.
Would your rotors be on the thin side perhaps? In other words, the wrong ones, poor quality or not up to specs?
If so, the pistons & pads would never have a fighting chance of clamping onto the rotor.
What you described is called bias braking. Other vehicles may have a bias valve to tweak line pressure front to back. On C3s there is no adjustment and no bias.
The proportioning valve is for emergency braking in the event of a failure.
Another thing, great DOT3 flow at the rear bleeders still does not confirm adequate line pressure. It's been said the brake-lines receive 400 lbs of pressure.
There is no way of checking that pressure at the caliper, that I know of.
One of the happiest days of my mechanical career was the day I threw my Motive power bleeder in the dumpster. Whoever designed that latch chain set up and surgical rubber hoses that break down from brake fluid should have made one more to shove right up his ***. Sorry for the rant. You can do a quick test of your brake light circuitry by grounding the wire that attaches to the proportioning valve switch with the ignition key in the ON position and the emergency brake OFF. This will tell you that the circuit is good (or bad) but it will not tell you if the switch is bad or out of position. Then, with the electrical connector removed get a test light, connect the clip end to a positive power source and touch the switch terminal with the test light. If the light comes ON your switch is not centered. If the light does NOT turn on your switch is properly centered and your problem is elsewhere.
Cheers, Greg
I love my Motive. But you must throw the bird feeder chains away. Not the tank.
Make your own Master cyl lid with hose connections and use a mans clamp to hold the lid on.
Works like a charm.
Is your proportioning valve original? Sometimes the wrong valve is installed after failure. The 1981 needs a true proportioning valve, unlike the older vet's that only have a light switch to light the brake failure light.
Latch chain? I threw that away immediately. Use a big C-Clamp, especially if are only using the Motive for air pressure, and need to keep filling the reservoir. I have a big quick-clamp now that works even faster.
Here's the description of the combination proportioning valve from my 79 shop manual.
Well my parking brake does light up the warning light but I have not tested the valve switch as it’s a pain to get to. But I’ll pull it if need be. And yes I get flow when having someone depress the brake pedal with the bleeder open. From that ‘79 manual, is that saying to recenter the valve you need to apply 450lbs of line pressure??
I believe the valve is original. If I can’t find a quick fix, I’ll pull the unit when I’m rebuilding the front suspension soon. Just was hoping it would be an easy fix…
Well my parking brake does light up the warning light but I have not tested the valve switch as it’s a pain to get to. But I’ll pull it if need be. And yes I get flow when having someone depress the brake pedal with the bleeder open. From that ‘79 manual, is that saying to recenter the valve you need to apply 450lbs of line pressure??
I believe the valve is original. If I can’t find a quick fix, I’ll pull the unit when I’m rebuilding the front suspension soon. Just was hoping it would be an easy fix…
If you have flow with someone pressing the brake pedal, the valve is unlikely to be the problem. Especially if your "Brake" light is good in the other configurations.
Latch chain? I threw that away immediately. Use a big C-Clamp, especially if are only using the Motive for air pressure, and need to keep filling the reservoir. I have a big quick-clamp now that works even faster.
Here's the description of the combination proportioning valve from my 79 shop manual.
I thought I was looking at my setup,,,,, exactly how I do it,,,, same clamp.
If you have flow with someone pressing the brake pedal, the valve is unlikely to be the problem. Especially if your "Brake" light is good in the other configurations.
No. That's strictly an assist.
You have rear brake line low pressure somewhere.
About resetting the Brake Warning dash light & Prop Valve. A quick jab or two with all bleeders closed will reset the switch 99% of the time.
If even one of the six bleeders is open, obviously you will "trigger" the switch that there is a fault in the system.
I would bet a Chili-dog, fries & shake its not the Prop Valve, its not the booster and not likely the caliper (but maybe)
My money is on a tired-out Master, more specifically a worn out piston seals on the rear of the rez.
Like stated earlier, just because you have some pressure at the rear bleeders, does not guarantee 400 lbs of pressure is at that location.
Last edited by HeadsU.P.; Mar 22, 2022 at 10:00 AM.
Another thing, great DOT3 flow at the rear bleeders still does not confirm adequate line pressure. It's been said the brake-lines receive 400 lbs of pressure.
There is no way of checking that pressure at the caliper, that I know of.
Another tool to get! On the older C3's without a true proportioning valve the pressure will be the same front to back. The differential pressure front to back is created by different size caliper pistons.
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Brake issue - proportioning valve?
