F-150 LEV question.

[351M]

Hey all, I was looking at a 2000 F-150 with a 5.4 that the dealership said wouldn't run on propane(they had 2) now my question is what typically goes wrong with the factory propane canversions? I had it running(away from the dealership of course) and it seemed fine but there was a leak when the fuel system was opened, like when you first turn the key on and while running. Sounded like it was line pressure but I can't find exactly where it was. Any ideas on this?