Back flow Problem?

[StarckMad]

I just finished replacing the copper plumbing in my '74 Argosy 26' with PEX. New pump, new bathroom sink, new commode. Turning the pump on I found one leak and fixed it. I left the old pressure regulator and back flow valve(?) in place. Then noticed the pump was pushing water out the hose connection on outside of trailer. Is there I fix for the existing preventor or should I replace the whole shebang? (Sorry photo is upside down.)

[HiJoeSilver]

It would likely cost you more to get parts than buy a new one. At least for the regulator. Not likely parts available for check valve, the guts are pressed in when built.
The regulator may be good.
It looks like on the wall side is an inline check valve. Replacing the check valve will stop the back flow. You can put a check valve in further downstream in the inlet line if it's easier to access.