If it's the same unit I'm picturing in my head, then get a standard thread 1/2" PVC cap.<br />If not, take the cap off, and go to a boatyard, and see if they have one.<br />Another atempt would be the hardware stores.
im in Scotland we get a thing here in a tube the plumbers use fir sealing toilets its like a claer rubber soultion in a tube try your local iron mongers its clear silicon put that on the threads and pray
The plastic seal washer behind the cap is probably missing. Sometimes you can get them to seal good enough by simply wrapping the cap with plumber's tape before you screw it in.
