With a diaphragm valve, usually the only reason you'd have a leak would be from dirt or other foreign matter keeping the diaphragm from sealing properly. With the truck aired out, and the tank empty, remove the 4 bolts that hold the two halves of the valve together, take out the rubber diaphragm and spring, and clean the heck out of everything. Check for any tears or damage to the rubber, and reassemble- no grease or oil necessary.
If this doesn't solve your problem, email me at
phil@fbirides.com and I'll have you send that valve back for warranty replacement, since we handle the Blitzluft warranties directly.
Good luck!
-Phil at FBI