Guys: Soliciting any and all input. I have had a fuel pressure anomaly for about 6 weeks now. The engine runs great and I have just over 18 hours of flying in it thus far. Worked off many gripes but, an elusive fuel pressure problem remains. When I reduce the throttle at altitude the fuel pressure rises. This is when I am at cruise, say 9000' at 2400 RPM and 22". I can control the fuel pressure rise with the prop and mixture of course but, if I don't adjust either of those I get a climb in fuel pressure to about 30 PSI. I now have the fuel pressure sender remote and isolated. I have also changed the sensor. Last Saturday, I hooked up a temporary line see photo to the fuel pressure sender and ran controlled air in at 5 psi increments. The pressure gauge I had on the line was even with all my Dynon readings on the panel. Ran it up and down 4 times and allowed it to hold for about 20 minutes on the last one. I am reading unmetered pressure off the test port prior to the manifold now. I thought that it might be pressure induced by altitude? On the ground it works as advertised; reduce throttle and the pressure comes down. I tried 4500', 2500' and 1500' and the pressure would always rise. I am beginning to think I have a return problem either at the bypass or in my plumbing? The lines were installed per the manual though and I am not sure where else to look? At cruise power it flies great and I have had no evidence of the engine running rough. Any thoughts to you with greater knowledge. Many thanks.