Mate just above i have quickly edited the post ,my bad its normally the FEED line you clamp., sorry about that. So prime the pump and system and then clamp the feed line. Normally if the system is equipped with a check valve its on the feed line. So lets say you clamp it, and the gauge stays good then the good side is leak free. If it drops when clamped then the engine side is your fault area mate. If it stays good then the fuel is leaking back pre rail mate, so yeah please try clamping the feed and give results mate
cheers
Jmac