Put the cap on right after you fill it up. Don't sit there and watch it throw the coolant back out. That's the normal ebb flow of the coolant in the system. Every radiator I've ever filled would do that if you sat there long enough with the cap off and watched the coolant level in the radiator. The system builds pressure as the coolant expands that's why you need a good cap or you will flow over into the overflow tank.