I've found the green 22psi cap is very hard on the system, it caused a two year old head gasket to leak externally, the coolant was weeping through the gasket!
Replaced with the 10psi black cap, too low, as soon as it vented, the cooling system went into boil.
I'm going to try the white 14psi diesel cap, anyone else tried any other caps?

Regards, Andrew.