well thats new to me i have no idea where do i start. any tests? 
disconnect the vac line to the booster at the manifold and put a cap on it. try to make the truck die. *NOTE* when you do this, you will NOT have power brakes. the pedal will be very hard.
also, try sparying the line with starting fluid or carb cleaner. if you hear the engine pick up while spraying a certain spot, there is a leak there.
