... I put a very small chamfer on the joint where the valve body meets - top piece only. Then used a urethane o-ring from a paintball tank that you can get on ebay for $8 for 100 and reinstalled.
The chamfer helped the o-ring fit to the tube and it hasn't leaked since. It was converted to a regulated HPA bottle running at 1100PSI 2 years ago and it still doesn't leak.