Your main check valve is leaking. Wear on the ball, or wear on the needle, or inadequate spring tension. (Pop off should be set to 10 PSI with gas) There just isn't any other place that will leak statically.
I'm more familiar with the HL, but same animal. Without a fuel shut off valve, it's not "if" it will leak, it's "when."
Diaphragm doesn't have anything to do with that, it just pumps.