The rear of the booster seldom leaks unless the diaphragm is junk. On the front of the booster, where the master cylinder bolts on, there is supposed to be a black neoprene seal that seats down in the hole. It's called a "face seal." You can purchase them new from a dealer, as long as you call it by the right name, they should be able to get one for you if it's missing or torn up.
There are only 2 sizes, one for early model boosters and one for the later units that uses the plastic reservior "quick take-up" type masters.
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David
newbie #4153
