A buddy of mine has a 91 (1ton?) with a cummins turbo diesel.He recently put all new brakes on it.With the engine off it has a great pedal and it stays up fine,when he starts the engine and trys to hold the brake the pedal slowly goes to the floor.It has a vacuum pump and it seems to be working but I don't know much about the vacuum systems on these trucks.What should I check to resolve this problem?

 

I would try bleeding the brakes or look for bad vacum check vavles.