I had the same thing happen on two of my hot rods...the Steel braided fuel line from the fuel pump back to the tank would collapse from the pump pulling a vacuum. On the other rod the pump was mechanical on the engine, and the vacuum pulled by the pump would eventually collapse the liner in the hose.

I replaced it with steel braided Teflon (PTFE) hose and fittings, and never had the problem again.

Also, fluid filled pressure gauges mounted in the engine compartment will also change readings as they heat up. Not in your case since the engine gets starved for gas.

BR, Mark