Black Maxx Owners Manual can be had here.
At $5...I suppose that is a deal. These were WalMart-esque guns.
The leak could be a couple things. Any idea where specifically the leak is coming from?
Fixes could range from teflon tape on the threaded fittings to new o-rings to simply greasing the already in place o-rings.
Best advice would probably be to just save up for a working, if not better gun. Can't really go all too wrong with a $5 gun for tinkering though...