Air bubbles in fuel

1986 GTi with DX engine. Having run fine before winter, now refuses to start. It appears there is air in the fuel lines leading to the injectors. Having pressurised the system cracking one of the injector feed lines on the fuel distributor reveals air bubbles and petrol. Obviously this means there isn’t sufficient pressure at the injectors tor the engine to run. I’ve replaced both fuel pumps and the filter. There’s no evidence of any fuel leaking from anywhere and I’m at a loss to see how air is getting in. Fuel tank is almost full. Help.

How you checked the swirl pot inside the tank including the mesh filter for any blockage?