Master Cylinder - Cover Leaks

I have looked inside the car and no there is no fluid inside the car nor is there any leaking inside the rear or front brake area, just in the front of the cylinder. When I pump them up I can go out and there is a leak in the front only and go back inside the car and have kinda of weak pedal. Maybe I should take it off and sand down the top, is there any way of doing the sanding while still on the car, maybe drain the cylinder stuff rags and them do a master cylinder bleeding...