Chris,

Pressure in the radiator could be from the head gasket leaking. There should be very little pressure below the boiling point

Are there any traces of oil in the overflow tank or radiator. You might have to take off top radiator hose to check for traces of oil.

Usually when you are loosing coolant, I mean internal to the engine not an outside leak, its something wrong with the head gasket or a cracked head.

One other check I just thought of is to run the engine and look for any bubbles in the overflow tank or an increase in the level of the coolant when the engine is revved up.

I'm not sure how a head gasket could be installed incorrectly but its possible.

If you have a mechanic that works on your cars, why don't you tell him to fix it and see what he says?

P E H