Never ending master cylinder leaking issue

[Steve73QMach1]

RacerX hit the nail on the head with his post on the vacuum booster as being a possible issue. I had the same master cylinder leak issue back in 1981 as you describe. Went through about 5 different master cylinders - all leaked at the mount point. Finally went to a junkyard and pulled a vacuum booster from a 73 Cougar. Once I installed that on my mustang, the leaking master cyclinder was no longer leaking. I drove that car daily for the next 5 years with no brake issues.

 

[72HCODE]

I put a rebuild booster on a couple years ago, if it leaked it would have to be the shaft seal.

I'll have to look into it again shortly once I finish my backing plates.