You likely have a HyrdoBoost system. The booster most frequently leaks from the cup seal for the piston and out the front of the unit. Leakage becomes apparent at the gap (drain) between the booster and rear of the maser cylinder. Repair is fairly easy, and replacement is just a little easier. Search for "hydroboost" and you'll likely get some hits.
The system will still require bleeding after the replacement.