1977 454engine fuel pressure drops to 0 in 15mts

[1977ChevyC20]

I have a 1977 Chevy truck with a 454 motor and a Holly carburetor, I installed a fuel pressure gauge and notice the fuel pressure ( starting at 6 psi ) slowly and gradually drops to 0 psi in 15 minutes when idling
I have installed a new fuel pump and filter and the same thing happens
I have removed the fuel caps when the fuel pressure is at 0 psi and there is no difference
The truck has duel tanks, I am wondering about the Left, Right tank switch or if there is a air leak ( nick or hole ) in one of the pipes ( if the fuel pump is sucking in air leading to decrease of fuel pressure )
Before i drop the tanks and renew the fuel line at the rear tanks I am wondering if anyone had any ideas
At 0 psi the trucks engine revs up and down a bit and gets slower, I suppose it would eventually stop
I can blow down the inlet pipe near the oem ( stock standard ) mechanical fuel pump and i can hear the bubbles in the left side fuel pump