Brakes?
 

i replaced the brake lines and the vacumm booster on my 96 cummins and my vacum reads 27 inches when driving youll have one good push on the pedal and the brakes will stop the truck then the rest of the time u push the pedal the pedal gets stiff and u have no brakeds until u wait like 30 seconds and vacum builds back up also i rebuilt the vacum pump and still have problem and the the four wheel drive acts up any suggestions??

sounds like a vaccum leak or the veins in the pump are weak

check above the 5th valve cover back... right were there is that weatherstrip that seals the hood to the wiper cowl... thats were my vac line cracked. i lost 4x4... braking sucked... and heat would only work on defrost... fixed that.... problem solved!