I bought a seven year old E-M Cobra earlier this year. Early on I noticed the Tilton hydraulic clutch master cylinder was leaking and replaced it. When I disassembled the old unit, I discovered the aluminum cylinder was very worn from the in/out action of the actuating rod. About a month later, the Tilton brake pedal began to drop when holding pressure and I knew the brake master cylinders were going too. I came to believe that aluminum master cylinders designed for race applications wear out faster than steel cylinders, so I went looking for a suitable substitute. I ended up buying and installing steel master cylinders from CNC. So far, so good. You can see them at:
http://www.pitstopusa.com/CNCSTEELMASTERCYLINDERS.asp
Incidentally, my car has 1987 Corvette suspension and brakes. I used 5/8" bore master cylinders.