truck running brakes bleed down

The master cylinder was actually the problem to start with!! You did not properly bench bleed the new one!! Now the system has a bunch of air in it as answer #1 said!! After you bleed about 8 cans of fluid through it, brake pedal should start to feel better, provided there are NO leaks!!! ...ANY vehicle will have some pedal fall when engine is started... been that way since 1928, when the first vacuum booster was put into use!!!

You gotta bleed it good still get air inside your brake system