I thought the seal on mine was the problem and changed that 'O' ring. I thought the leak had been cured. However, when I sold the gun, the buyer told me it was still leaking
Very fortunately, he was a thoroughly nice bloke who also knew a thing or two. It turned out to be a leak from the reg!
If you look at the reg on those. It's joined halfway along its length. The guy separated the two halves, resealed and reassembled and all was fine.
If it still leaks after the new 'O' ring. Strip the gun so that the action and air cylinder are out of the aluminium, with the air cylinder attached. Put some soapy water around the joint of the reg, if it bubbles, it leaks.