Replaced shaft seal by first snipping off the old push-on nut, then sliding the old seal off the shaft. Using a small screwdriver and a little plunging action on the shaft itself helped to wedge the seal out of the housing. Turns out the original (factory) seal had been installed backward, which explains the leak. Very important to orient the seal as described in the instruction sheet with fluted edge toward pump housing. Sliding the new push-on nut down the shaft and locking it in the shaft groove takes some patients as it is very tight. Used needle-nose pliers to apply an even push on the nut while holding the top of the shaft with pliers to avoid pressing the shaft too hard against the other side of the plastic pump housing. Didn't want to take the chance of puncturing or cracking the plastic.