Diagnose Fuel Restriction by Bleeding Line

What happens here

To diagnose the fuel delivery problem further, the owner demonstrates bleeding the fuel line at the injection pump. When the bleeder screw is loosened, instead of fuel being pushed out (indicating a healthy fuel supply), air is noticeably sucked in, creating a vacuum. This observation strongly suggests a blockage in the fuel line leading from the tank to the pump. Based on this, the decision is made to remove and inspect the fuel shut-off valve at the bottom of the fuel tank, as it's a common point for debris accumulation.

Goal: Confirm a blockage in the fuel line