Its not an issue with the winter or summer seals as you would find the gun leaking in cold weather if you had the summer seals in.
Its either the valve assembly or possibly a mucky or corroded hammer? The wrong o ring on the hammer would also cause the same sort of problem. Plenty of people have put a bottle o ring on the hammer which is a fraction too small.
If you really get stuck then call Powerball on 01252 408550.
edit, oops, supposed to be a reply to the Inferno question.
Its either the valve assembly or possibly a mucky or corroded hammer? The wrong o ring on the hammer would also cause the same sort of problem. Plenty of people have put a bottle o ring on the hammer which is a fraction too small.
If you really get stuck then call Powerball on 01252 408550.
edit, oops, supposed to be a reply to the Inferno question.