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Old 01-01-2005, 03:54 PM
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Brake pedel weak

Guys' I have installed a 95 S-10 booster and MC on my 37. I have a problem I have never encountered before. When I pump up the brakes they seem fine and are firm , but while I hold them down after about 20 seconds they sink to the floor. Also it takes about three pumps to get firm, the first pump is soft and goes pretty far down.

I have bled them about 4 times and tried two different master cylinders. Any ideas?

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Old 01-01-2005, 04:01 PM
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Is the master on the firewall? The car probably still has air in the lines. Also the flex lines could be soft and expanding. If they haven't been replaced for awhile you might want to do so. Are the wheel cylinders in good shape? I've seen air sucked in aroung the wheel cylinder seals.

The sinking pedal sounds like a master cylinder problem. Are you sure the other masters you tried are known good parts?
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Old 01-01-2005, 06:04 PM
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brakes

Check wheel cylinders. Master cylinder sounds like its bad. Even if you purchased a rebuild from a parts store. I have had rebuilt calipers leak right out of the box.
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Old 01-01-2005, 06:56 PM
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I went on the MP Brakes web site and they said to block off the distribution block with bolts to isolate the master cylinder from the front calipers. I tried it, and the pedal is solid. So this leads me to believe it is not in the MC. However it seems like if the calipers were bad, they would be leaking. Right?
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Old 01-01-2005, 08:29 PM
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[i] However it seems like if the calipers were bad, they would be leaking. Right? [/B]
Probably. There is air in the system or the flex hoses are bad. One other thought, do you have disc on the front and drums on the back? Is the master cylinder plumbed properly with the disc portion of the MC feeding the disc brakes?
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Old 01-02-2005, 09:31 AM
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After talking to few more people, I think I have a bad master cylinder. These things are pretty cheap. I'll change it today and post if it works.

While I'm at it, I think I'll change the hoses.
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Old 01-02-2005, 05:11 PM
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Well I don't think it was the Master Cylinder. I have now tried four different ones and they all do the same thing (two were brand new).

BTW I think I have it plumbed correctly, but just to be sure anyone with a 90-95 or so Chevy S-10/Blazer, can you confirm for me the front reservoir is plumbed to the big line and feeds the back brakes, and the back reservoir (i.e. closest to the firewall) has the small lines and feeds the front brakes.

Last edited by RickB1B; 01-02-2005 at 05:18 PM.
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Old 01-02-2005, 05:53 PM
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Pretty sure it's the other way around...at least my 83 is...............

Are you using a Blazer proportioning valve?
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Old 01-02-2005, 06:32 PM
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Your symptoms sound like there is still air in the system.

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Old 01-02-2005, 08:30 PM
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I fought the same problem on my 74 Vette and it was the right front caliper not bleeding all the air out properly. I tracked it down by plugging off the rear (no pedal) hooked the rear back up, then plugged the fronts ( I had a pedal) and then plugging of the left front (no pedal) and then unplugging the left and plugging off the right (had a pedal). I took the caliper off and apart and cleaned the passages with air and out came a chunk of rubber o-ring. Vettes use a four piston split caliper so there is a bleed passage for each half. The obstruction prevented one side from bleeding its air out. Put it back on bled it and all is well now.
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Old 01-04-2005, 05:04 AM
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Chebby usually plumbs the front port to the front brakes.
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they seem fine and are firm , but while I hold them down after about 20 seconds they sink to the floor.
This sounds like a leak to me. Maybe air combined with a collapsed hose...
But I'm open to anything. Maybe just having the drum port plugged ito the front discs could do it....(?)

Those step bore masters can be a pain to bleed sometimes. Use single stroke method or pressure bleed. Wait 15 seconds between pumps on the s-s method. Re-bench bleed when you swap lines, and bleed at those lines when re-installed.

You can isolate front from rear using your blocked port method, just block one circuit. This might help you isolate the problem.

What are your wheel brakes?
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Old 01-04-2005, 03:11 PM
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Is the pushrod correct?If it is too short you will not get maximum travel inside the master cylinder even though pedal travel is correct.
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Old 01-04-2005, 05:19 PM
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I went out to the junk yard today and looked at several other cars and trucks. The large tube is in front and goes to the back brakes. So I at least have this correct

I tried using several different metering valves including the one which the master cylinder came off and they all had the same problem.

Don't know about the pushrod since the Booster I used was missing one, but I got another off the same kind of truck. I also checked with several others and they were all the same size, so I don't think that is it.

What I'm wondering now is; is there a difference between MC's with anti-lock brakes and one without. Could it be anti-lock brake cars have a mechanism to release pressure located in the MC.
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Old 01-04-2005, 07:15 PM
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You never did say..................Are you using a proportioning valve?
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Old 01-04-2005, 07:46 PM
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Yes I am using the same proportioning valve that was on the car with the MC. I have also tried others just to make sure it wasn't bad.

I'm starting to think the only thing left is air in the system. Although, I have bench bled it no less than 7 times and have put over a gallon of fluid through the system, maybe the old method of bleeding just isn't good enough.

Do the hand pump vacuum bleeders work?

For what it's worth, every component in the system is new, except the proportioning valve, and there are no leaks.

Last edited by RickB1B; 01-04-2005 at 07:54 PM.
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