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Discussion Starter #1
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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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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Discussion Starter #4
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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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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Discussion Starter #6
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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Discussion Starter #7 (Edited)
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.
 

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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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Shop Owner And Troll Hunter
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Your symptoms sound like there is still air in the system.

Troy
 

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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.:thumbup:
 

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Chebby usually plumbs the front port to the front brakes.
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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Come Home Safe Soldier
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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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Discussion Starter #13
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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Discussion Starter #15 (Edited)
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.
 

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I had the same problem

I went through the same problem on my 33 Plymouth. Weak pedal after replacing the MC.

I bled the turkey four times with no luck. I also forgot to bench bleed the new MC.

Finally I used a bleeding kit with a container to prevent any back pressure air to get in. Bled the system at all four wheels. It looked like there was no air bu the pedal firmed up and I've had no problems since.

Air will get in the system and its a b***h to get out. Hang in there and keep bleedin !!!
 

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Shop Owner And Troll Hunter
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By pumping the pedal and getting a firm pedal and holding it, then it slowly going down is an indication of air still in the system. If it pumps up then the rod should be alright. Make sure the calipers are on right with the bleeders up on each side, if not they will never purge all of the air out.

Troy
 

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The M/C for ABS and nonABS are of the same design on the S10. Both are 24x36mm step bore made to match 63mm front "low drag" metric calipers and 3/4-7/8 rear cylinders. It is also designed to work with an S10 combination valve. The master contains no residual valve. The largest resivoir should go to the fronts.

Only the ABS control valve has the ability to bypass fluid.

The combination valves may differ, as they do between disc/drum and disc/disc, and different weighted vehicles.

A vertically mounted valve has a different bleeding procedure than a horizontal mount, as it can trap air.

A disc/drum valve can contain a valve for the fronts which holds off pressure until the drums catch up, and shouldn't be used on a disc/disc system.

The difference in the rear brake part of the valve is how much pressure it restricts and when. There also might be a residual valve in it....

Sorry I have no advice. Just info of which you might already know.
 

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I forgot something else about the step-bore master. I dont know if this will help with your diagnosis or not...

The check valve (QTU valve) inside the master closes at about 100psi. While the valve is open, the large (1 7/16") bore delivers high fluid volume with little pedal travel. After the valve closes, pressure is delivered by the small (15/16") front bore.

The step bore is deigned to operate low-drag calipers without giving a mushy pedal.

I think this negates the need for a front hold-off valve in these systems, as it would probably screw it up.
 

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Discussion Starter #20
This is great information and may help me out. I noticed the other day I had the old style front calipers. I then bought new ones, but the parts store gave me one of the old ones and one of the new (metric) ones. When the pressure comes off the brakes I noticed the old one is dragging pretty good.

I will update this and see what happens.
 
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