The truck has new brakes and drums on the rear. The front pads and rotors are almost new. It has a new master cylinder and brake booster.

When the engine is off, I have a full brake petal. When I start the engine and press on the brake, the brake petal will slowly go all the way to the floor.

Can someone help me with this problem??

 

Sounds like a vacuum problem to me I had the low vacuum sensor go out on mine it is located next to the vacuum line tee on the fender wheel but don't go out and buy one unless you are sure your pump is putting out good vacuum and you have no leaks, or air in the lines.

If pump is working then run a line straight from pump to booster and if works then you have a leak somewhere else. Just check it this way don't drive around town like this.

1. Make sure everthing is bleed really well if you just put the brakes and everything on.
2. Check your vacuum and see if it is an acceptable level

Someone more knowledgable than me will come along an give more suggestions.

 

A vacuum problem won't cause the peddle to go all the way to the floor. Only a problem in the hydraulic system will do that. Usually the master cylinder is leaking internally, the system leaks somewhere or the system hasn't been bled properly and there is still air in it somewhere. With the engine on the vacuum gives you a great deal more force on the master cylinder, so any problems will show up much more quickly.

DO NOT, DO NOT, DO NOT drive any truck where the brake peddle sinks all the way to the floor, as it is an accident waiting to happen.