Lift pump Mystery

Ive got a 97 12v that Ive extended the frame on to fit a 12' Douglas service body. Everything from behind the cab is new including the axle and frame. I ended up using a fuel tank out of a 2010 Ford that is mounted just behind the axle. Ive redone the entire low pressure fuel system and return with ptfe AN hoses and steel hard lines. I have a 50micron water separator and a 2 micron baldwin filter in the pressure side before the p pump. The air dog Raptor 150 is mounted at the tank and has a 1/2" feed into a draw straw into the tank. its about 3' of suction hose from the end of the draw straw to the Raptor. I used the original ford tank return line.

The truck makes 30psi when i click on the pump and humms away quietly like it should, but when i start the truck the pressure drops to 15-20 psi and the Raptor gets really loud like its cavitating. The pressure steadily drops throughout the day for a couple hours untill there is no pressure.

This is the third pump that ive tried (pretty sure its not the problem) and its been similar with all of them. Ive tried opening the tank fill cap to see if its vapor loc but no change. Any ideas??

Has to be a restriction in the inlet. I'd pull the tank and the sending unit and look at the pickup tube good.

I was using the ford pickup assembly which is 3/8”, and I thought the same as you, so I just put in a 1/2 draw straw and 1/2” hose to the pump inlet, it’s cut at an angle on the bottom so it can’t suck to the bottom of the tank, and same thing???

What about your return line?