Leaking master cylinder or power booster?

When I purchased the car the front reservoir towards engine was down about 1.5 inch I added fluid and it held for about a month then down about the same. I could not find a leak at the wheels so I thought it could be a steel line. No brake fluid on floor untill today so I checked area and found it seems to be right at the spot where the master cylinder attaches to the power booster. I know the master cylinder can leak or is it where the master cylinder attaches to booster there must be seals on both parts? Is it the master cylinder or the booster?  The leak seems to from the booster.  Any help would be appreciated so I have an idea on what cost may be. My brake pedal through this has been very hard could a faulty check valve cause the leak? 

The master cylinder is leaking. You'll need to either replace it, or have it rebuilt.....but a replacement is the better choice, imho. These aluminuminum cylinders are hard to repair once they get corroded.
If it has been leaking long enough, the brake fluid can wind up filling(or partially filling) the booster, which can cause a harder than normal pedal.

Thank you the brake pedal has been hard the day I purchased the car.  I will set up an appointment to put in a new master cylinder, and I hope the boaster is still good.  I really appreciate the help.  I almost feel like doing it myself but something always goes wrong. Take care!!!