Convert siphon feed 'paint cup' spray gun to pressure feed?

[harringtondav]

I'll be applying a lot of drywall primer and finish paint at our river house project. Enough that I will break out my old Sears pressure pot spray system. The original gun is getting iffy and I'm wondering if I can convert my Campbell Hausfield siphon gun to the pressure feed hose.
It would seem as easy as removing the paint cup mounting assembly and attaching the paint supply hose. My paint supply hose fits the CH gun. But the original Craftsman gun required a screw inserted near the paint inlet to convert from siphon to pressure feed.
Any thoughts or experience with this?