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  #16 (permalink)  
Old 12-17-2018, 09:30 AM
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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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  #17 (permalink)  
Old 12-17-2018, 10:44 AM
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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.
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Old 12-17-2018, 11:03 AM
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Quote:
Originally Posted by 64nailhead View Post
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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Old 12-17-2018, 06:05 PM
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Okay - strike two. Nothing broke or gummed up. The actuator inside moves what Id refer to as freely. It will move slowly with 60-70psi of air. Also no leaks. Onto the master cylinder.


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  #20 (permalink)  
Old 12-17-2018, 07:17 PM
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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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  #21 (permalink)  
Old 12-17-2018, 07:37 PM
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Originally Posted by johnsongrass1 View Post
You could try too attach the lines together and see what happens before going through all the work of replacing the MC again.
This should result in a solid pedal with very short travel?

Also, I tore apart the original m/c. There was nothing wrong with it, much like the proportioning valve. I'm onto rigging up my pressure gauge in different locations to the rears. I need to make a piece of NiCopp line that I can hook to a bubble flare union. I can see a visit to the parts store in the near future.

Any chance I could have a caliper binding causing this? Both rear calipers are originals from '90 but it's a Florida car with less than 80k for what that might be worth.
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Old 12-17-2018, 08:07 PM
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Yes to the pedal but it also tells you that the MC is moving and returning fluid.

Binding is possible. So is a loose wheel bearing or a caliper rocking around or a flexing caliper mount.
Sometimes the pedal will feel hard because the piston is bound up and a release of the pedal relaxes the bind enough to retract the piston further than it needs so the next time it take two or more pumps to move the piston enough. Keep in mind the pads are always rubbing the rotor on a disc brake so the piston only need to move 1/16" to apply enough pressure on the pad/rotor contact.
At $20 each, I'd change them and dress the caliper brackets if your are at all in doubt.
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Old 12-19-2018, 07:21 AM
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Centering tool

It could be the combination valve is going off-center while you're bleeding the brakes, it happens sometimes. Get one of the centering plugs and use while bleeding.
INFO;
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Old 12-19-2018, 07:23 AM
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Is this a new setup?
Has it worked properly in the past?
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Old 12-19-2018, 06:57 PM
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Quote:
Originally Posted by lmsport View Post
Is this a new setup?
Has it worked properly in the past?
It has worked properly in the past and this is NOT a new setup. It is the stock F-body setup.

A brief synopsis : brakes worked great for a little over a year until this last spring. The front brake hoses were holding the front brakes on(original hoses). Changed hoses, pads and calipers and all was well - drove less than 200 miles. This fall the brake light would come on occasionally and has gradually worsened to the description in the title including the brake light on at the same time as the 1st pump.

I had one suspect steel line on the left rear, so I changed it tonight - no difference and identical complaints. Two things - I did as Tony described - stab the pedal very hard. When that is done the brakes work fine and no brake light. I'm testing this on jack stands with the motor off, the motor running, and the motor running in gear. Normal brake application in all three states results in brake light and no rear brakes until pumped up.

My son was manning the brake pedal this evening and he mentioned that pumping the brakes quickly is difficult because the pedal doesn't return quickly as compared to everything else he drives.

The only parts that haven't been changed are the rear calipers and the booster. From my understanding of booster function I don't see how this could be possibly caused by the booster. Nor do I see how it could be a caliper or calipers causing this.

I'm open to all suggestions. Ole Nolan - describe the proportioning valve centering locking mechanism to bleed the brakes. I could remove the switch and jam something in the slot to hold it in place while bleeding, but geesh, I've never heard of anything like that.
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  #26 (permalink)  
Old 12-19-2018, 07:03 PM
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Quote:
Originally Posted by OLNOLAN View Post
It could be the combination valve is going off-center while you're bleeding the brakes, it happens sometimes. Get one of the centering plugs and use while bleeding.
INFO;
https://www.youtube.com/watch?v=qPLaPv8DITE
This video identifies that I could have air in the combination valve or my front brake spring has weakened allowing the front brakes to apply prior to the rear - correct? FWIW, I'm not buying that I have air in the valve as I've ran more than a quart of fluid through the rears during bleeding and about half of that was done via gravity bleeding. But if you think my thought process is all wet, then let me know.

Forgot to mention that it still, repeat still, is only letting the brake pedal travel with 3-4 inches of the floor when bleeding the rears - the pedal will not go to the floor when bleeding the rears. Maybe this is normal for 4 wheel disc as I'm usually working on a disc/drum setup.
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  #27 (permalink)  
Old 12-20-2018, 08:17 AM
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Quote:
Originally Posted by 64nailhead View Post

I'm open to all suggestions. Ole Nolan - describe the proportioning valve centering locking mechanism to bleed the brakes. I could remove the switch and jam something in the slot to hold it in place while bleeding, but geesh, I've never heard of anything like that.

Did you watch the proportioning valve video? They show it and explain it. It's just a threaded plastic plug with a nipple on the end.
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  #28 (permalink)  
Old 12-20-2018, 05:20 PM
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brake light means big pressure difference, if no air in system then the master is not putting out the correct pressures. do you have access to brake pressure gauges, we use them for race cars to get an idea on brake bias.
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  #29 (permalink)  
Old 12-20-2018, 07:14 PM
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I think it's fixed. The issue appears to be a simple problem that I have never encountered for the mere reason that everytime we, son and I, jump into a new project we always put new brake hydraulics on every wheel end and I change every brake line, hard and soft. This GTA is not mine nor my son's, and it's a rust free unit which is unlike most all of the rust belt beauties that we usually build/work on. So I ***/u/me that there wouldn't be a corrosion issue.

I pulled off the rear calipers and found the sliding sleeves that ride on the o-ring were rusted and not sliding. The passenger side was not moving at all and the driver side was better, but not moving properly. The result is that the pads were too far from the rotor after the brakes were released causing an excess amount of fluid needed to have them engage the rotor. This caused the brake light from the combination valve switch and no rear brakes on the 1st pump. A couple of minutes with a scotch roloc disc and a little caliper grease was needed.

Tony's post about panic pushing of the pedal and Olnolan's video put me onto it. And lmsport's last post describing the excessive bias was what got me thinking. When I was 'panic stop' pushing the pedal with no light and good rear brakes the light bulb went off in my head. Only took me a week or so - hehe.

Thanks for the help everyone - it was entertaining, educational and interesting - at least to me lol.

Tomorrow it's supposed to rain in monsoon fashion - 3" or so - and be in the 50-60 degree range. Which should do it for a temporary cleaning of the salt off the roads. Perhaps I can get in a couple more burnouts and brake tests.

Thanks - Jim
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  #30 (permalink)  
Old 12-21-2018, 05:08 AM
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Somehow it alway's goes back to the basics. This thread reminds me not to over think a problem and look at all the standard parts first.
Glad you found and corrected the issue.
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