On my drive to work this morning I didn't have any power assist on the brakes until I had driven a bit. Do these trucks have any external check valve that could be causing this? It's like it had to warm up for the power assist to work. I figure either the booster is bad and some rubber parts eventually expanded due to the heat and sealed thus the brakes began to work or If there is an external check valve it was leaking and it took some time to build up vacuum. I don't hear any hissing or anything.