Instructables
video Replacing Hand Pump Cup Leathers
This is an inexpensive pump but there is no reason it can't perform well.  In this video I show how to change out the crapy factory rubber cup gasket for a far more durable leather one.

Great. I especially liked the fact that you explained everything you did by saying the problem, what you thought could be done to solve it, trials of how to do it right with the tools you needed and your expectations. This video is an excellent didactic tool for those who want to get into the know of fixing anything. It is the approach that you have that is precious. I know the feel of not succeeding in doing a repair job so when I saw the water come up I almost jumped for joy ha!