You may also want to put a bit of grease on the actual threads before you install the back cap.
One trick I also like to use is to press the back into the threads gently, then turn backwards about a half turn to make sure I have the threads lined up properly, then screw it in. If you encounter a lot of resistance then something is wrong.
The reason I do this is because the threads are sharp so if you cross them even a bit it can cut an oring. You want it(the cap) to go in smoothly on it's own and not force it too much.
ETEK 3LT, with 14" Ti boomstick and cure 3 + bolt F/S
Vanguard creed eyes, membrane, bolt pin, detents, gaskets, schraders, orings and more F/s[/color]
Old school angel poppets/cup seals(LCD and A4/speed), spring detents(not sure which models) F/S