Before you give up on this, you could try some loctite 515 sealer on all of the machined surfaces and o-rings. Everything must be super clean and dry! I use this stuff at work on chilled water equipment to take up any remaining leaks in the o-ring seal on the refrigerant side of the machine. These things get leak checked to 500 microns(almost a perfect vacuum, around -60lbs or 30 inches of mercury) and this stuff seals up any imperfections. I don't see why it would not work on your seal if it is an o-ring, but if it is a flat rubber gasket, it may squish out when you start tightening it.
PS-If you are not sure if you have the right stuff, it is purple and kind of smells like grapes....seriously!(you will keep smelling you fingers, no matter how disgusted you are by the smell) Should be available at most HVAC supply places, or they can point you in the right direction.
Here is what it looks like:
http://www.all-spec.com/1/viewitem/5...=yahooshopping