In reply to afghanidan:
You can use regular o-rings (oil resistant ones). Just replace them when they look old or leak. Buy a pack of assorted sizes and you are set for a long time.
Regular lubrication of the o-rings with some spray lube keeps them in good shape. Also keep the pump leather in good shape with vaseline or leathergrease.
There is not much more to maintain, except cleaning the noze and do not loose parts.