odd brake problem.. bad master cyl, or bad booster?

im picking up an 87 MJ, and its got an odd problem. when the truck is off it has pedal pressure. when you start it, it loses pedal pressure. take the cover off the master cylinder, and you can see fluid moving when the pedal is pumped.

im thinking it might be the booster.... but brakes are not my forte.

:helpme:

Bleed the system, check for leaking cylinders at the rear, etc. Booster sounds like it's working fine. Start bleeding furthest wheel from the MC.

markw said:

Bleed the system, check for leaking cylinders at the rear, etc. Booster sounds like it's working fine. Start bleeding furthest wheel from the MC.

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i'll know more when i get it home, thanks for the idea.

its actually been sitting for about a year, so it could be any number of things. :dunno: start with the cheap stuff i guess

thanks for the reply

markw said:

check for leaking cylinders at the rear .....

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X2. My '89 is notorious for the rear cylinders going bad.