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Old 07-17-2021, 03:22 PM
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Default 69 Bonneville Brake problems

Has anyone had problems with new or rebuilt master cylinders?
I had a problem with the front brake locking up, proportioning valve was stuck, dirt in lines. Cleaned it and flushed lines, worked until this spring.
Calipers sticking, replaced with rebuilds, lost all brake pedal, plus old proportioning valve started leaking, I replaced it a unit from an A body, didn't help then with a Wilwood manual adjustable unit. Still no brakes on front, just rears. Appears to be a bad master cylinder. So far I have had 5 different ones on it from new to rebuilt and one from a car that was working. Pedal goes to the floor on all of them. No loss of fluid, no leaks. Have bleed over and over , no better. The rear brakes will lock up, no front. Replaced calipers with a 2nd set, same result. However If I clamp off the front hoses, I get full pedal ! If i take the clamp off the left side I get good pedal, it still locks up the rear first, but it will stop the car. If i clamp the left and release the right side, no pedal, car wont stop.
All these always lead back to a bad master cylinder, but everyone I try has the same result, low pressure seams to be the factor. Has anyone had this problem? Any suggestions....

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Old 07-21-2021, 02:43 PM
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I'd post pictures of your setup, especially the lines. Sounds more like something with the lines or the delay or proportional lines. maybe things are hooked up wrong.

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Old 07-22-2021, 05:05 AM
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Lines are all as they came from Pontiac. Only change is a willwood manual proportioning valve that didn't solve the problem

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Old 07-22-2021, 06:29 AM
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Quote:
However If I clamp off the front hoses, I get full pedal ! If i take the clamp off the left side I get good pedal, it still locks up the rear first, but it will stop the car. If i clamp the left and release the right side, no pedal, car wont stop.

This would seem to indicate the problem is between the clamp and the right caliper?
Does fluid come out when opening the bleeder on that caliper?
When pressing brake pedal with clamp on and brake bleeder open, release clamp. What does it do?



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Old 07-22-2021, 09:06 AM
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Are the bleeder valves on top or bottom?

Had to ask because I've seen it happen more than once.

Wrong caliper for that side puts bleeder valve on bottom. Which makes it impossible to bleed the air out.

Clay

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