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Discussion Starter · #1 ·
As the title describes and at the same time the brake light will come on. Also the brake pedal is a little deeper than normal. If I pump the pedal twice or more, then all is well.

My guess is the proportioning valve is not functioning correctly, but I already guessed at a master cylinder and was wrong. So I figured I'd ask the experts before I guess poorly ..... again lol.


Vehicle is a '90 GTA Firebird with stock 4 wheel discs.
 

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You've been around for awhile, so I'm sure you've checked the M/C fluid levels. Sounds like the prop valve is doing it's job, alerting you to an imbalance. Have you checked the rear calipers for signs of leakage? Might try bleeding the rears...

Russ
 

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Discussion Starter · #3 ·
Funny you have mentioned bleeding the brakes :). I've bled them all. The rears though do not bleed correctly. The pedal travel stops before the pedal hits the floor when bleeding the rears. When bleeding the fronts it bleeds normally.

I wonder if the hose to the rear has crapped out (collapsed)?
 

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I would have guessed the M/C also. Curious, how do you know the rear brakes aren't working?
 

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Discussion Starter · #5 ·
I would have guessed the M/C also. Curious, how do you know the rear brakes aren't working?
Car sitting on jack stands with tires removed. I have my son pump the brakes once and I easily turn the rear axles with a pry bar in the wheel studs.
 

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Discussion Starter · #6 ·
So I'm convinced the issue is the proportioning valve. I can't seem to locate a new one that uses the metric fittings. So, 1st question, anyone have a source for these? And second, is it possible to find parts to rebuild this one? Last, any chance that this one could just be 'gummed up' and tearing it aprt and cleaning it will resolve?

I have little to no experience with these things other than I've never seen one fail. But from watching videos on youtube I see that many older ones, pre 1980's, can be cleaned and rebuilt with kits that are available.
 

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When bleeding the rears, does the fluid come out with the same pressure of flow as the front? Does it squirt or dribble?
 

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Discussion Starter · #8 ·
When bleeding the rears, does the fluid come out with the same pressure of flow as the front? Does it squirt or dribble?
No, it does not come out of the rear bleeders as it should. It does more than dribble, but not much. I'd say somewhere between a squirt and dribble. I'm experienced at bleeding brakes and it's obvious that the rears are not correct. If I try to bleed them on the first pump, it will barely dribble and the pedal doesn't drop. If they are pumped up, then the pedal drops, but not to the floor. When bleeding the fronts, everything acts normal when bleeding.
 

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Have u broke it lose at the hard line and flex line at the rear end and seen how it flows under pressure? That flex line might be bad.
 

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Discussion Starter · #12 ·
The process is two fold, check the flex line and proportioning valve.
Define how to check the proportioning valve - lol. I've gone through a 'centering process' with it a couple of times with no avail.

I mentioned previously that I have no experience tearing apart proportioning valves, but I believe that I'm getting closer to getting some experience.
 

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I've seen few proportioning valves go bad, but once it was narrowed down a bad one, everything else fell into place and I got a steady stream to all bleeders.


In recent years I've seen TOO many bad masters out of the box, but doubt this is your case. If you can't find a new one, are used ones available? Like putting on dirty underwear after a shower but a good diagnostic tool.
 

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Define how to check the proportioning valve - lol. I've gone through a 'centering process' with it a couple of times with no avail.

I mentioned previously that I have no experience tearing apart proportioning valves, but I believe that I'm getting closer to getting some experience.
To see if sufficient pressure is at the connection of hard-line and flex-line. If sufficient pressure is not there then it is either the proportioning valve or the hard-line. If hard-line is suspect take it off at valve and blow through it.
 

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You may have to take off the master cylinder and see if it is returning to the stop. I have seen new ones bind and not return, but not all the time. If it is not returning it will not feed fluid into the chambers. Make sure there is enough clearance on the brake push rod and enough return spring pressure to bring the pedal back enough also.
 

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If when this happens as you said in your first post, the pedal goes further to the floor, I would not suspect a blockage anywhere. In any scenario I can think of, a blockage would give you a higher than normal pedal. I have gotten bad master cylinders, new from an aftermarket vendor or rebuilt.


Most of the time when I've had a bad master, if I jump on the pedal really fast, it usually works that time. I'm guessing it stretches the seal a bit and it *catches* the fluid. Also suspect bad ones seem to leak down easier with light foot pressure, not hard.


Try trouncing on it and see what it does. And how long after it grabs does it have to sit until it doesn't grab again?
 

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Discussion Starter · #17 ·
It seems to be a consensus that perhaps I bought a dud m/c. So, since the new one reacts exactly hte same as the old one, I'm going to disassemble the old one and see if the rear brake piston is travelling/locating correctly.

Still no recommendations to tear apart the proportioning valve though? It's amazing that there is little to no available info about taking one apart (smh). Also, I'll try slamming the pedal to see if it makes a difference.
 

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It seems to be a consensus that perhaps I bought a dud m/c. So, since the new one reacts exactly hte same as the old one, I'm going to disassemble the old one and see if the rear brake piston is travelling/locating correctly.

Still no recommendations to tear apart the proportioning valve though? It's amazing that there is little to no available info about taking one apart (smh). Also, I'll try slamming the pedal to see if it makes a difference.

If you can sacrifice it, pull it apart. It may just need a cleaning and new O-rings.
 

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Discussion Starter · #19 ·
IMG_0764.jpg IMG_0765.jpg

Okay - strike two. Nothing broke or gummed up. The actuator inside moves what I’d refer to as freely. It will move slowly with 60-70psi of air. Also no leaks. Onto the master cylinder.


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You could try too attach the lines together and see what happens before going through all the work of replacing the MC again.
 
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