My thought is, if the pump is working and then cutting out, it's not a relay. Relays usually are not the problem. I am also thinking 7psi sounds really low. I am thinking it should be around 20-40 psi. Sounds like your pump is going out. But to answer your question about finding the relay. It should be located under the hood in the factory fuse/relay box. You'll have to check your owner's manual for the exact slot or download it online to find the slot #.
Scott -- Commercial Point, Ohio
2004 Damon UltraSport
Freightliner Chassis, Cat C7