Then thats why, your relay needs to have two power feeds, one is the one which will feed the pump, (Should be identifiable by having thicker wiring) and the other is the feed to the relay itself which energises it, you then have an output from the relay which will be your pump feed and the other will be an earth.

As a test you could bridge the feed you do have (What will be the pump feed) to the other terminal where the second live you are missing should be, do so with the ignition ON this should power the relay and pump priming should commence. You then know it's all working ok so you just have to find why the second feed is missing.

Martin