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Dgas 06-12-2007 08:11 AM

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??????? :confused:

Frisco 06-12-2007 08:29 AM


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.

Dgas 06-12-2007 04:26 PM

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,

Bigmac26 06-12-2007 05:16 PM

Had the same problem with my 36 cpe - added a return spring to help it return - works great now

Dgas 06-12-2007 07:56 PM

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.

Dgas 06-19-2007 09:47 AM

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!

Dgas 06-19-2007 10:20 AM

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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