[wizard]

Loose.
If it's tight pressure will build and prevent water from moving back and forth.

An alternative you might try is use a radiator cap rated at less pressure than your regular cap, the lower the better, but I would still be careful.

How is water routed to this tank? Normally , they are fed from the bottom so water moves back and forth and not air. ( am assuming you have a hose connected at the other end to the pressure bleed off port on the primary filler neck.)
Some pics would helpful.