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-   -   No rear brakes on 1st pump (https://www.hotrodders.com/forum/no-rear-brakes-1st-pump-518731.html)

64nailhead 12-10-2018 07:53 PM

No rear brakes on 1st pump
 
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.

S10xGN 12-11-2018 08:05 AM

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

64nailhead 12-11-2018 01:59 PM

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)?

55 Tony 12-11-2018 05:17 PM

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

64nailhead 12-11-2018 06:49 PM

Quote:

Originally Posted by 55 Tony (Post 4641709)
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.

64nailhead 12-16-2018 07:20 AM

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.

Fang16 12-16-2018 07:33 AM

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

64nailhead 12-16-2018 08:20 AM

Quote:

Originally Posted by Fang16 (Post 4642729)
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.

Fang16 12-16-2018 09:45 AM

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.

64nailhead 12-16-2018 02:47 PM

Quote:

Originally Posted by Fang16 (Post 4642759)
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.



New - changed it about a month ago.



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Fang16 12-16-2018 08:49 PM

Quote:

Originally Posted by 64nailhead (Post 4642827)
New - changed it about a month ago.



Sent from my iPhone using Tapatalk

The process is two fold, check the flex line and proportioning valve.

64nailhead 12-17-2018 03:41 AM

Quote:

Originally Posted by Fang16 (Post 4642889)
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.

PHWOARchild 12-17-2018 06:52 AM

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.

Fang16 12-17-2018 08:46 AM

Quote:

Originally Posted by 64nailhead (Post 4642905)
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.

Fang16 12-17-2018 08:54 AM

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.

55 Tony 12-17-2018 09:30 AM

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?

64nailhead 12-17-2018 10:44 AM

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.

PHWOARchild 12-17-2018 11:03 AM

Quote:

Originally Posted by 64nailhead (Post 4642967)
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.

64nailhead 12-17-2018 06:05 PM

2 Attachment(s)
Attachment 439003Attachment 439005

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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johnsongrass1 12-17-2018 07:17 PM

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

64nailhead 12-17-2018 07:37 PM

Quote:

Originally Posted by johnsongrass1 (Post 4643063)
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.

johnsongrass1 12-17-2018 08:07 PM

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.

OLNOLAN 12-19-2018 07:21 AM

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;

lmsport 12-19-2018 07:23 AM

Is this a new setup?
Has it worked properly in the past?

64nailhead 12-19-2018 06:57 PM

Quote:

Originally Posted by lmsport (Post 4643353)
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.

64nailhead 12-19-2018 07:03 PM

Quote:

Originally Posted by OLNOLAN (Post 4643351)
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.

55 Tony 12-20-2018 08:17 AM

Quote:

Originally Posted by 64nailhead (Post 4643525)

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.

lmsport 12-20-2018 05:20 PM

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.

64nailhead 12-20-2018 07:14 PM

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

Bob37pont 12-21-2018 05:08 AM

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.

lmsport 12-21-2018 07:39 AM

the mid-eighties to about 90 model f-bodies were campaigned for bad calipers....people with auto trans never use their e-brake which means the ratchet mechanism in the rear calipers never gets exercised which is how the calipers adjust themselves as the pads wear. GM replaced the calipers with units that had coated parts in the ratchet.

johnsongrass1 12-21-2018 09:14 AM

Quote:

Originally Posted by 64nailhead (Post 4643777)
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

Hey! where the credit for post #22? Oh it's like that huh? bahaha! Just teasing! It's good to hear it's working for ya.

64nailhead 12-21-2018 11:53 AM

Quote:

Originally Posted by johnsongrass1 (Post 4643853)
Hey! where the credit for post #22? Oh it's like that huh? bahaha! Just teasing! It's good to hear it's working for ya.

I forgot to mention that I did that and it worked - pedal was rock hard. It was an eay way to confirm the master cylinder was good. Too bad I didn't do that before I gutted the original.

johnsongrass1 12-21-2018 11:57 AM

I take stuff apart all the time just to see how it works. Usually learn something like I shouldn't have cause it won't go back together.


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