As long as the neck of the radiator is sealed then all the pressure/vacuum is acting on the overflow tube. A pressure cap with the lower seal removed still seals the top of the neck.
You should always start with the radiator full to the top and the tank 1/2 full. It will eventually equalize to a level doesn't overflow and is air free.
Last edited by Jim Sears; 03/21/2017 3:45 PM.
