First thing to check would be the fuel accumulator.
It's under the car, by the fuel pump.
It's there to keep the fuel pressure up for a while after the engine is shut off, to prevent hot start problems.
Next would be the check valve in the fuel pump, it can leak pressure after the engine is shut of.

Hook up a fuel pressure gauge and check how fast the system pressure drops after you shut of the engine.

__________________

76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.