C3 1970 Brake proportioning Valve Problems
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Matt,
You would have been better off starting a new thread. Not likely get an answer from 5 yrs ago.
The 3/8? I think you might mean the maximum distance a screwdriver is inserted when bench bleeding?
According to Lonestar, no more than 1 & 3/8 inches or you void the warranty on a new / rebuilt master.
And yes, you can fabricate two brake-lines for testing the MC. That test can be done on the car. And that isolates the master, all by itself to verify where a failure issue is located.
Prop Valves seldom fail. Boosters seldom fail. But the other stuff . . . . . . . ?
Click on my avatar and look for photo album - brake bleeding.
You would have been better off starting a new thread. Not likely get an answer from 5 yrs ago.
The 3/8? I think you might mean the maximum distance a screwdriver is inserted when bench bleeding?
According to Lonestar, no more than 1 & 3/8 inches or you void the warranty on a new / rebuilt master.
And yes, you can fabricate two brake-lines for testing the MC. That test can be done on the car. And that isolates the master, all by itself to verify where a failure issue is located.
Prop Valves seldom fail. Boosters seldom fail. But the other stuff . . . . . . . ?
Click on my avatar and look for photo album - brake bleeding.
Intermediate
Joined: Mar 2024
Posts: 27
Likes: 3
Thanks HeadsUP,
I did as suggested and blocked off the brake lines on the master and I can now confirm that the master cylinder is fine. 100% firm pedal, not moving an
inch when the brake lines are blocked.
I also confirmed that it is the front brakes causing the issue.
I bled these, by putting a clear plastic tube from the bleeding nipple directly into the master cylinder reservour and pumping brakes until no sign
of any bubbles whatsoever. The I closed it all up and started the car. Guess what, pedal still sinks right down to the floor.
No leaks anywhere. It is driving me nuts.
Best Regards, Matt
I did as suggested and blocked off the brake lines on the master and I can now confirm that the master cylinder is fine. 100% firm pedal, not moving an
inch when the brake lines are blocked.
I also confirmed that it is the front brakes causing the issue.
I bled these, by putting a clear plastic tube from the bleeding nipple directly into the master cylinder reservour and pumping brakes until no sign
of any bubbles whatsoever. The I closed it all up and started the car. Guess what, pedal still sinks right down to the floor.
No leaks anywhere. It is driving me nuts.
Best Regards, Matt
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Well, Matt. I hope you re-bled all 6 again.
Sorry, but . . . .
After using the fabricated master cyl test lines, it is impossible not let air in the system when reinstalling the old brake lines.
A bubble is guaranteed to enter both front & rear lines.
The fact that the master held pressure means the issue lies elsewhere.
And I take it you applied pedal pressure for several seconds during the test without any movement?
I do question whether you purchased the correct master. Design changes did make some yrs not work properly.
And I do still question the booster rod gap measurement even though the master holds pressure.
Sorry, but . . . .
After using the fabricated master cyl test lines, it is impossible not let air in the system when reinstalling the old brake lines.
A bubble is guaranteed to enter both front & rear lines.
The fact that the master held pressure means the issue lies elsewhere.
And I take it you applied pedal pressure for several seconds during the test without any movement?
I do question whether you purchased the correct master. Design changes did make some yrs not work properly.
And I do still question the booster rod gap measurement even though the master holds pressure.
Last edited by HeadsU.P.; Sep 8, 2025 at 07:27 PM.
Intermediate
Joined: Mar 2024
Posts: 27
Likes: 3
Hi HeadsUP,
I actually just did the same test again, just to be sure.
I blocked the front, so only the rear brakes would be applied and although there is a little
play, once the brake engage it is super firm and does not move a bit, not even after several seconds.
I noticed the brake light went on when I held the brake down with the front brakes detached. I guess that is the proportioning valve,
so I may need to re-center that (I heard you may get away with just pushing hard on the pedal a few times
when all brakes are hooked up).
I am pretty certain it is something with the front brakes. I wonder if it can be something as trivial as the
rubber hoses that are expanding due to age? I will check that next.
I actually just did the same test again, just to be sure.
I blocked the front, so only the rear brakes would be applied and although there is a little
play, once the brake engage it is super firm and does not move a bit, not even after several seconds.
I noticed the brake light went on when I held the brake down with the front brakes detached. I guess that is the proportioning valve,
so I may need to re-center that (I heard you may get away with just pushing hard on the pedal a few times
when all brakes are hooked up).
I am pretty certain it is something with the front brakes. I wonder if it can be something as trivial as the
rubber hoses that are expanding due to age? I will check that next.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
The preferred way to use Test-Lines on the master is to block BOTH systems. Front / Rear.
Your way of testing does not isolate the master, all by itself. You still had a brake-line incorporated.
And yes, the Prop Valve can be reset only if the entire brake system is closed up. A few quick jabs of the pedal.
By the way. Part of the reason to fabricate master test lines is no harm will come to the masters outlet ports using fittings.
Under no circumstances should anyone ever use a bolt to block a master port.
The master is an iron casting and has a flare machined inside for the brake-line. A bolt used as a plug will destroy the casting.
Your brake-line hoses maybe due for a swap.
I would open a front bleeder, then just snug it. Pump the pedal numerous times and immediately open the bleeder again.
If DOT3 dribbles out: Likely ok.
If DOT3 shoots out: Rubber hose is now a One-Way Valve. Replace.
If no fluid, could be caliper.
Your way of testing does not isolate the master, all by itself. You still had a brake-line incorporated.
And yes, the Prop Valve can be reset only if the entire brake system is closed up. A few quick jabs of the pedal.
By the way. Part of the reason to fabricate master test lines is no harm will come to the masters outlet ports using fittings.
Under no circumstances should anyone ever use a bolt to block a master port.
The master is an iron casting and has a flare machined inside for the brake-line. A bolt used as a plug will destroy the casting.
Your brake-line hoses maybe due for a swap.
I would open a front bleeder, then just snug it. Pump the pedal numerous times and immediately open the bleeder again.
If DOT3 dribbles out: Likely ok.
If DOT3 shoots out: Rubber hose is now a One-Way Valve. Replace.
If no fluid, could be caliper.
