Remove the master and check the inside of the brake booster. More then likely the rear seal of the master is leaking and its entering the booster. You'll need to replace both as brake fluid supposedly eats the seals in the booster.
Good luck.

 

I'd replace both master and booster. chances are that the internals on the master are worn enough to warrant replacement and the cost of a rebuild kit is usually about the same as a rebuilt master.