The bolt holding the bottom of the mast cup is screwed into a brass insert molded into the hull. The plastic caps are just covers. When you do the "vacuum" test, use soapy water with a sponge to see where air escapes. Expect some out of the rudder handle and the front hatch, and a little out the rudder tubes and up/down pull tubes, but nowhere else (that I can remember). Unless you find a crack, any other leaks should be able to be stopped by tightening the screws or bolts.
Thanks for the help! I found the culprit, looks like it's all related to the cracked cover. It was really bad today, I must of had two gallons at the end, and I bailed at least a gallon at sea. But that might be because the rudder pin popped and I had to lay out off the back to replace the pin, temporarily submerging the rudder line holes.
Look carefully and you can see a crack coming out from either side of the mast cup.
Close up of crack on the left side.
Close up of crack on the right side.
Cracks in the plastic cover. If you look carefully you can see stresses proceeding out from the cover in the hull, probably where the cracks are on the inside.
I put 3 cups of water in the hull and then watched for leaks around the cracked cap, look at this nice solid drip!
Anyone ever seen this before?