I started having the typical low/no heat output problem from my heater:

1. I had the heater core flushed (no help)
2. I had the restrictor valve replaced (it had some seepage around the collar anyway....some help, but still not back to normal)
3. Finally I had a pressure test done to the cooling system to find that the radiator cap did not hold pressure. W/the new cap the heater works fine now.

hope this helps someone.

Word of warning: Some places that flush your cooling system/radiator use a corrosive solution to remove internal scale and rust. This may accelerate aging of your water pump.

 

Thanks! My heater was a bit whimpy during the last Florida winter (the couple days I used it. LOL!) and will definitely give this a shot