That sounds like the fuel accumulator. It failed on my 190E 2.6 a few years ago. I started a couple of threads one of which was a destructive analysis report that revealed cracks in the diaphragm that allowed fuel to short back to the fuel fed hose from the tank resulting in no fuel pressure buildup.

The symptoms were intermittent hard starting (long cranking time) that got progressively worse. I developed a simple on the car test to determine if the device had failed.

The weird part was that there were zero replacements in the USA both Bosch and Mercedes Benz. I finally got one from a Mercedes Dealer in Kansas that had one flown in from Germany. My 2.6 was down for two weeks, but I have other cars.

The accumulator holds about 20 cc of fuel under pressure, so if your fuel tank is very low and slosh uncovers the pickup, the system should maintain pressure for a few seconds before the fuel covers the pickup again.

Duke