i cant get heat to work in my 84 diesel four door when i am completly stopped. some times i have to turn the dial to make it work. but it allways works when stopped. my problem is it wont work at all when im driving. it doesnt seem that the aux pump or the heat valve by the fire wall function when i am driving. but they do when i am parked. the heater has worked a few times while driving, but 95% of the time i get no heat while driving. only while the car is completly stopped. i can get lots of heat while at a stop light. but as soon as i start rolling the heat stops working. is there a vaccum leak? please help