O-rings reliable, yes, the piston rod should be polished, with the polishing going lengthwise, the bore should also be as smooth as you can get it. The ring groove on the piston should be 50% wider than the ring, and remember you can't compress rubber.
The valve should seat on the sharp edge, you need a spare ball, place that on the hole, and with a brass rod on the ball give it a light tap with a small hammer, use another ball for the valve, the one you hit may be distorted. You might need a D bit to drill the seat flat, or I may be misstaken but sometimes the seat is at the top of a very shallow cone. Ian S C