I found the PT Cruiser 2.4 to be very easy to fill. I simply filled at the radiator, started her up, topped it off just a hair, then filled the tank to the hot level, never had an issue whatsoever, almost the easiest one I ever did. Check the tightness of the hose clamps all around, shouldn't have been touched to change the pump itself, but you never know. Stock ones are the squeeze clamps, just make sure they are intact and look to see if any of them were moved. Also, turn the heater on full blast, sometimes when they drain the heater core becomes an air bubble and very slowly it empties of air, usually it only takes a couple minutes of full flow to purge it.