Reservoir cap is supposed to be loose. You're on a goose chase there.

Fill the radiator completely, engine cold, start it with the radiator cap off and rev it. If a bunch of bubbles constantly come out, head gasket is probably leaking. This test is more sensitive if you put your hand on top of the radiator instead of the cap (engine still cold) and watch for bubbles in the reservoir.

Next take reservoir cap off, wash out the hose from the radiator to reservoir with water (so you don't get antifreeze in your mouth), put radiator cap on and blow gently into the reservoir end of the hose. Should be airtight. Fix any leaks such as at the hose nipples or top gasket in cap.