[Jack21]

Unless you're running something really radical for a cam, there should be plenty of vacuum for the heater valve. Simple test is if you have enough vacuum for the brakes to work, the heater valve should also. You have a vacuum leak or defective valve.

That being said, you may not want a vacuum operated water valve for the heater at all. It's either all on (too hot), or all off (no heat). A manual valve operated by a cable will regulate your heat better, and any auto supply store has them for a couple of bucks.

I'm about a 10 - 15 minute drive from you (small world, eh?). Come see how mine's setup.