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Thread: Weak Master Cylinder???

  1. #1

    Weak Master Cylinder???

    I own a 67 Chevelle backhalved Pro street. It has 4 piston wilwood brakes at all four corners. The master cylinder is an original GM style not vacuum assisted, which looks like it has seen better days. When I drive it, I have to pump the pedal to build up any psi to just stop. The car does have line loc. I bled the brakes and still have the same issue. Before I start changing parts like an idiot, I rather ask your guys opinion.

    Does line loc affect regular braking like this and do I have to bleed the solenoid???Or bleed in a different manner???

    Can a stock GM mastercylinder open all 16 pistons?

    I have seen some cars run a dual mastercylinder set up, pros and cons???

    Will a Wilwood master cylinder solve the problemss?

  2. #2
    Senior Member RACING JUNKIE
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    Is the master higher than the calipers?If it is mounted real low(under the floor board...)the fluid will drain back to the master and you will have to pump it back out to stop.Been there before myself Also make sure you use residual valves.Basically a one way check valve.Disce require the 10 psi res valves.That might help too.And ,no the line lock won't affect you,or need bleeding.It's only function is to valve off the line to hold the fluid pressure until you release it,then it's wide open.That won't be your problem.
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  3. #3
    Senior Member RACING JUNKIE
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    After you pump up the brakes, does the pedal remain hard, or does it slowly go to the floor? If the pedal remains hard, I would say your master cyl. is not moving enough fluid. Those 16 pistons have a lot of area, so the master cylinder will need to move a lot of fluid when you stomp the whoa peddle. If you dont already have one, I would install a proportioning valve, or an adjustable bias valve.
    suicidebomb
    No matter where you go, there you are.
    **IN GOD WE TRUST**

  4. #4
    Senior Member RACING JUNKIE
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    when you pump the pedal to get pressure, does the pedal stay up or does it fade?if it fades, lok for a leak in the lines , hoses,if the m/c is located on the firewall look behind it, even the floor board inside. if it stays up id say still have air in the system.i worked on a customers vette awhile back. same type problem.pedal stayed up but had to pump the hello out of them. we bleed them brakes so many times.every thing i checked seemed good.filled up the m/c and went to lunch. when we got back i put the m/c top back on, which i left off and went to bleeding one more time.on the last wheel i was geting fluid first the the biggest brake fart i have ever seen and then we had brakes.i would have never thought that amount of air could have been in that system, but there it was or should say there it went.im not saying thats your problem but id start there. four wheel disc can be a real pain in the butt to bleed sometimes.if we can help in any way pm me and i will give you our phone # good luck.

  5. #5
    Senior Member RACING JUNKIE
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    If the pedal remains hard and has no fade its definetly needs burping again, and again, till you get it all out. All them pistons can trap air and can be stubburd to bleed.
    The method that works best is to hook a small rubber hose off the bleeder screw and then submerge the hose in a bottle of brake fluid ( a old coke bottle works good but any clear bottle will do. Next open the bleeder screw and then slowly cycle the brake pedal all the way then release repeating several times till you have no air bubble present ( requires another set of eyes) in the bottle.
    I would save your money on a wilwood M/C and get a later model GM quick take up master cylinder, they work just as good or better as the pricey wilwood units.
    JMO> Charles
    ***IN GOD WE TRUST***

  6. #6
    Senior Member MASTER BUILDER
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    Is your master cylinder the old GM style that came on the 67 Chevelle with standard brakes (narrow top) or is it the fatter top master which was used with the later Chevelles with disc brakes? You need the larger one at the very least, because the disc brakes do need a larger volume of fluid.

    But I saw this problem once before on an older Chevelle and it wasn't the master cylinder. The one that I had problems with was because the Wilwood calipers had been just bolted on and were not centered and square with the rotors. The caliper pistons retract easily, so that you don't have drag and this was causing the brakes to need to be pumped up to set the pads back against the rotors. You need to make sure that the calipers are centered with the dividing line in the center of the rotor from top to bottom.

    Bill M

  7. #7
    The car has a double chamber fatty master cylinder in the original position on the firewall. It is all rusty and the cap seems to be not sealing well. I wanted to change it anyway because it looks like crap compared to the rest of the engine compartment. I think the previous owner swapped the master cylinder with this one. When I did try to bleed the brakes, the pedal would get firm but then it would get soft again. No leaks anywhere. When I drive it there is no psi on the first push. It takes 2 to 3 pumps to get some psi. It will stop the car at normal speeds. I can't imagine screaming throught the traps at 140 mph and trying to stop my Chevelle. The wilwood discs should stop on dime, I presume. I just thought aftermarket master cylinders created more hydraulic force than a stock one. The aftermarket ones don't seem to hold more fluid than a stock one although I have seen reservoirs for extra fluid. I am going to start with a rebuilt GM master cylinder and bleed a river throught the system.

    I really appreciate the feedback by all. I'll post my results.

  8. #8
    Senior Member RACING JUNKIE
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    It's not so much, how much fluid they hold, but how much they move. In other words some M/C have bigger pistons in them so with the same stroke they move more fluid. the downside is a bigger piston will take more effort from your leg.

    suicidebomb
    No matter where you go, there you are.
    **IN GOD WE TRUST**

  9. #9
    I think I found the problem. I was looking under the dash and noticed that the rod that pushes the piston into the master cylinder wasn't engaging until almost an inch of travel. I blasted it with some wd40 and will try to adjust tomorrow

  10. #10
    Just adjusted the rod under the dash today. Peddle is nice and hard (that's what she said).
    I can't test drive until later in the week but I am confident this was the problem. Thank You for all your feedback.


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