How is your brake pedal? Is it soft? If so your master cylinder either needs to be rebuilt or replaced. Or if your pedal feels firm, then the seal is leaking.
The brake fluid is most likely filling up your brake booster. It would be best replace the seal between the master cylinder and the brake booster. As well as remove as much of the brake fluid as possible and clean it out. Carry out this procedure as soon as you have a new seal, or master cylinder, as failure to do so can and will lead to failure of your brake booster.
You will need to bleed the brakes to do this procedure, dot 3&4 brake fluid. Build your self a pressure bleeder, and this job will be relatively simple.
I followed this guild to build my pressure bleeder:
The DIY $20 brake bleeder
FYI: A New master cylinder (only buy ATE (oem supplier) or OEM). Will come with a new sealing ring.