There is a Over Flow Valve (pressure control valve; controls the fuel pressure and fuel return from the IP) on ther rear top of the IP near the idle adjusting screw that has a hose going to the fuel return on the fuel filter. It is not there to stop fuel bleed back.
However, the fuel supply pump on the side of the IP has at least 2 one way valves.
As stated before it sounds like an air leak any where from the primer pump back (if you have one).
After that it is possible that the supply pump valves are not seating due deposits as the mesh on the little plastic filter is not that fine and over time stuff can build up.

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84 300D, 82 Volvo 244Gl Diesel