The accumulator keeps the fuel pressure up for a while after the engine is shut of.
This is done to keep the fuel from boiling in the lines when the engine is hot.
It's necessary to makes hot starts easier.

Testing?

Connect a pressure gauge and observe the pressure drop after the engine is shut of, but there are other parts in the system that can cause a pressure drop.

You could remove the small hose from the accumulator and plug it.
Now run the pump/engine.
No fuel should come out of the hose connection on the accumulator.
If it does, you have a bad diaphragm.

__________________

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.