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Old 06-12-2007, 08:11 AM
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pwr brake booster issues?

I changed the booster and MC in my 37 cpe yesterday,The MC was leaking brake fluid in back which ruined the diaphram in the booster-
I bought a new MC w the correct piston Dia. + rod hole (Deep) in the primary piston and a used 68 Chevelle Booster ,Now When I step on the brakes the booster energizes all the way ,locks the brakes up and stay's that way for a minute B4 slowly coming back up to normal and releases the brakes, I can release them manually w my foot,when I disconnect the vac source the brakes act as they should ????? Now what???????

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Old 06-12-2007, 08:29 AM
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Quote:
Originally Posted by Dgas
I bought a new MC w the correct piston Dia. + rod hole (Deep) in the primary piston and a used 68 Chevelle Booster
Master cylinders with the DEEP push rod hole are usually used with no power assist.

Master cylinders with the SHALLOW push rod hole are usually used with power assist.

Sounds like you have the wrong master cylinder for your application.
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Old 06-12-2007, 04:26 PM
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GM uses both long and short push rods on their Booster/MC's, this one has the long rod and the MC piston has the matching deep hole,
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Old 06-12-2007, 05:16 PM
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Had the same problem with my 36 cpe - added a return spring to help it return - works great now
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Old 06-12-2007, 07:56 PM
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just heard that the front brake line boss may have a residual valve behind the fitting bushing I'll see if it does, if not i'll try a return spring.

Last edited by Dgas; 06-13-2007 at 08:11 AM.
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Old 06-19-2007, 09:47 AM
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Update- changed the MC THREE times, all different ones none had a residual presure valve, finally got a the correct 1968 disc brake MC for my 68 Chevelle booster STILL goes to lock as soon as I start the engine and I have to release it w my foot ,added a heavy pedal spring and still locks, but returns quicker than B4 but still slooow, W/O vacumn it works fine, a bit of a hard pedal but fine . Now What!

Last edited by Dgas; 06-19-2007 at 10:18 AM.
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Old 06-19-2007, 10:20 AM
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one more ? Our auto repair guy says I can use a short shaft booster w a deep hole MC ,BUT not the other way around as the MC will be engaged but the longer shaft -true? I have the NEW disc MC but I am thinking its the booster odds are I'll not find a replacement JY booster as its a one year only booster (68 Chevelle),But I know of a brand new Delco Moraine short shaft booster I can get for 10 bux.
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