how do i keep expansion tank full?

It sounds like your C5 has excess pressure in the cooling system, if the coolant is still spewing out if you've replaced the cap. I would suspect cylinder head gasket problems which would cause this. Ask a capable garage to 'sniff' the air above the coolant with a gas tester - this immediately shows if there are any gasket leaks. Water pump leaks also aren't unknown on these.

John
December 2007