Sounds like the fluid is bypassing the seals in the Master Cylinder. You can buy a rebuild kit at any auto parts house and change out the seals. If you do this you will need to remove the Msrt Cyl from the firewall and rebleed the system when you put it all back together. It should just be a split ring that holds the MC piston in. Pull the ring and the piston should pop right out. Change the orings and put it back together.