1949 Merc not stopping after driving 10 minutes

we have installed a new dual master cylinder on my father in laws 1949 Merc, both residual valves installed, (master under floor) and after we drive it for 10 minutes his brakes start fading, so my guess is heat, but any suggestions for this problem would be great. thanks Neil my email is nmattison@aol. after it sets for 20 minutes then its fine again.

Sounds like air in the system still. Pull the master and do another full bench bleed and rebleed the wheels. You have to have the master off the car to bo a full bench bleed and get all the air out of the master.