Dante

I have a 98 CH613 with a 350 Mack Engine. I had to replace the radiator and now I can not seem to get the expansion tank to accept enough coolant to fill the entire coolant system.. but the radiator cap on the expansion tank is under a LOT of pressure but not HOT after a short drive… during this drive the engine keeps shutting down I ASSUME from either not enough coolant OR the engine is starting to get too hot….. HELP!!!!! How do I remedy this issue??? Thanks In Advance