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.