I also had problems with the stock o-ring leaking. I pulled out my assortment of O-rings and found three that easily stretched over the nipple of the pump and easily slipped in to the reservoir. With the O-rings stacked side by side, the reservoir has to be pressed on firmly in order for the c-clip to line up with the cut outs on the reservoir and the groves in the nipple. The pressure on the O-rings makes for a snug fit on the nipple and the reservoir. Hope this helps.
So you have no more PS leakage? Am I correct that the O-rings go into the reservoir with the C-clip to the "right"/away from the reservoir? At first I slid the O-ring onto the PS nipple with the C-clip to the "left" of the o-ring and of course it leaked. Im will update as I get some miles on the car. Thanks for the advice should I need to remove the reservoir again as I have a set of o-rings here! Mark