Because the metal filler neck of the S4 water pump provides the best spot to tap for my radiator fan switch, I'm converting from a S5 water-pump assembly to a S4 assembly.

My question is, with the S5 radiator (w/ the cap in it) and a filler neck w/ the over-flow top (which I'm going to run to the over-flow bottle) which cap (flat or pressure) would I want to use? The Pressure cap right?

If that's the case, is the pressure cap for the filler neck the same as the filler cap for the radiator? (I'd like to use an extra radiator cap I have [in new condition])

~THX, Colby (VERY poor college student)