...valve. Some are well made, some are not. A quality tank valve should only need a hand tight seal with a proper size o-ring. Third cause is using an incorrect size o-ring. IIRC, on most 6.8 and 9L tanks with U.S. 7/8-14 thread, a 212 o-ring is used. Chinese M18*1.5 tanks use a metric o-ring.