How about using a mechanical Tork brand timer like on a pool pump? It is two pole, will handle the amps of the motor, and it installed on the incoming power line, not the pressure switch wires. As long as you leave the hydrant open the pressure switch will be engaged. Then the Tork timer just powers the pump on and off according to your setting. I think those timers also have a way to just manually turn the pump on, bypassing the timer, for times when you need the pressure switch to work.