Brake Failure-75 Chevy Van

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  #1  
Old 10-08-06, 09:03 AM
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Brake Failure-75 Chevy Van

Does anyone have any knowledge of what is wrong when the brakes completely fail-pedal goes completely to the floor, and there is no fluid leakage. Is this the Booster or the Master Cylinder?
 
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Old 10-08-06, 09:21 AM
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Yes, it could be both the master cylinder and/or booster. Check for vacuum leak going to the booster. Have your tried bleeding your brakes?
 
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Old 10-08-06, 11:39 AM
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Contrary to other opinions, it definately is NOT the booster. If a booster fails, the pedal will get very very hard to press.


If you have absolutely no leaks and you have not repaired anything else lately that may have allowed air into the system, I would lean towards the master cylinder.

Master cylinders do not generally fail all at once. They are generally noticed as going bad by a simple test that actually mimics some normal driving that you may have noticed this problem before:

When the pedal is depressed quickly and quie firmly, it stays up high. then if you aplly slow gentle pressure to the pedal, it will fade away. Why this is is the m/c has cups inside that puch the fluid. When you press quickly and hard, they expand from the resistive pressure and seal against the bore. When you press slowly and gently, the resisitive pressure is not great enough to exp[and the cups and fluid bypasses them allowing the pedal to fade away.

One other thing to try is pumping up the pedal. If you can pump up the pedal and it stays up, post backwith this info and we'll look at a couple of other possibilities as well.
 
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Old 10-18-06, 05:24 AM
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bad master cylinder

this is an easy fix


bleed the new mastr real good before connectiong the lines

i always bleed them on the vehicle and have an assistant pushing the pedal while i hold my thumbs over the two ports until no air is coming out of them then reconnect the lines

good to go usually after that, sometimes no bleeding at the wheels is necessary after that


good luck
 
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Old 10-18-06, 06:48 AM
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When you say there is no fluid leakage, I guess you mean you don't see any brake fluid running down from inside any of the tires. Have you got down on the floorboard at the brake pedal and felt down there (where the master cylinder push rod goes through the firewall) for brake fluid? I've had brake fluid there when the master cylinder was leaking.

Assuming there is no brake fluid at the pedal, then I'd start wondering whether there is a problem with the ABS control unit. I've never had a problem with one myself, but that's the next thing that comes to mind.
 
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Old 10-18-06, 06:54 AM
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Old brake lines can rust and leak so be sure to look along the frame rail and make sure there are no leaks there.
 
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Old 10-18-06, 09:11 AM
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they said theres no fluid leakage so i assume they looked everywhere including along the frame and at all the wheels and they are probably not missing fluid i assume or they would have said probably,


hard to say though

if they arent missing fluid ever then its gonna be bad master

to test the booster start the engine and hten shut it off and let it sit about 5 minutes and then pull the big vacuum hose off the booster it should have alot of suction and be loud sucking noise

if it does then its fine

but thats not the problem here according to symptoms


GM boosters rarely go bad,
very very uncommonly do they fail on the old trucks and vans of GM




good luck
 
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