Do you have any pics? Is it a scratch built PUG or a palmerized PGP? Slightly different animals.
The cup seal is probably the biggest issue for leaks in a pump gun though if its coming from the lower tube I'd say you have a bad chamber plug face seal too. You are on the right track rebuilding the whole thing, and you should be able to do it yourself. I'd call PPS and see about ordering all the parts you want up front. I've tried a few rebuilds before buying them one piece at a time and paid a bunch of cash in shipping charges for an o-ring or two on each order.