I have been experiencing a very similar problem with a 30 PSI Shurflo pump that has been fitted firstly in my daughter's boat (now replaced) and latterly in my own boat. After checking very thoroughly for leaks, then changing the PRV, I concluded that the pump was building up too much pressure, thus opening the PRV. Changed the pump for an identical spare that I keep, problem solved. I now intend to service or replace the pressure switch on the faulty pump and hopefully then have a good spare. Good luck with yours.