Originally Posted by digidocs
If you haven't already, check the seals on the fuel valve. They aren't always designed to hold more than a few psi of pressure.
By golly, that was it! Thank you for your suggestion. I disconnected and capped the line going into the valve and pressurized the system, and it held 50psi solid for several hours. My primary concern is the lines down in the forward part of the tunnel, which I can neither see nor fix without great effort. The rest of them are much more accessible if I start having issues.