I have the measurements for the o-ring I used, they are, cross section of 0-ring - 3.5mm, od- 26mm, id - 19mm. The one that was in the pump is visibly smaller and fit loose in the outer tube. I think they must have used too small an o-ring when they assembled it.
Used the pump again last. Tried pumping a little faster and it seemed to work better but there were still some "dead" feeling strokes. Actually didn't take long to get from 150 to 190 bar, but to do it I had to tie some weights to the arm holes of a heavy suede vest (30lbs + my 130lbs was just enough)! I'm past my return window. If it doesn't get any worse than this, I can live with it. Wish me luck, Thanks, Frank