the walbro i can hear run when priming if i listen for it, the stock pump i never could hear. the walbro starts the car without a check valve, it takes an extra 1/2-1 second then before ont he stock pump if i dotn prime it. if i prime first it starts just like stock. With the check valve we have in now it starts like stock with no ill effects but im looking at others for the least restrictive of the bunch before i start sending them out.

the stock check valve is built into the end of the fuel pump outlet, not servicable.

your pressure at 55psi looks decent and yes with the quick pressure loss i totally agree the check valve has failed. i would swap in the walbro for a forced upgrade. then either grab an inline check valve or run with the walbro for a few days until we have the valves ready here also to complete the setup.

i can check on the price of a new OEM pump for you tomorrow thru dodge and mercedes but there will be some sticker shock to it