acm m500 shotgun

[ste887]

leaking gas as fast as i can fill it from the brass nut on the back, could someone wo has one please post a picture of thier o-ring set up, i believe i am missing one. there is a small one on the firing rod and one on the brass cap itself. but it leaks from this cap, tried silicon, worked for a few days then leaked just as bad, would lithium grease work better? and can i get a extra o-ring in there over the current one to stop the leak? or is it a replacement only job?
thanks
ste