Also by chance could the replacement cap be a partial pressure cap (vented). Liquid expands with heat (water in a closed system expands 2.5 % (non linear) of volume for every 100°F rise). If you're using a partial pressure cap...it will spew stuff until the cap shuts down the 'overflow' as designed. It will suck in air as it cools and shouldn't over flow again. An overfilled radiator will also exert more pressure than a cap is rated for until an equilibrium is reached between the expanded fluid and the caps spring pressure.
Try running to temp without the cap on. Don't refill. Put the cap back on, let it cool and run to temp again. Does it leak?
Regards,
RJ
