[kitcarbp]

Dont increase the pressure cap rating, keep it in the 10-12 psi range. Do you have the cooling system designed to have a large enough expansion tank volume? The over flow tank should not really be used as your expansion tank. If you have trapped air after the initial fill, you will notice the overflow tank liquid volume will drop after a few heat-cool cycles if the cooling system is desiged properly to vent entrapped air. Then you top-up the overflow tank to ensure there is always a minimum liquid level. Yes adding the small hole to the thermostat helps during initial fill and intial start-up to help move air to your surge tank/expansion tank so it can be purged to the overflow tank. I installed a thumb screw bleed valve at the highest point of my system (therm outlet neck) to aid in getting air out during the filling process.