check valve is another name for a none return valve built into the fuel pump its design was to hold pressure to the injection when motor not in use (over night) Bosch fuel pumps in the early days were noted for failure, same pump is used on the Merc and BMW

 

OK, guys... I can corroborate that access to the fuel pump in my '87 sedan is through an access hatch/plate in the for'd section of the trunk. It looks to be a pure PIA to get to and work on with the tank installed in the vehicle - not that I'm contemplating temporary removal to facilitate further "surgery"... I think leaving well enough alone is, at this juncture, the wisest course.

 

very similar to the Mercedes system, riser pump in the tank with sock filter lower main fuel pump then fuel pressure regulator, common fault on these is main fuel pump fail or its vibrating on the body new pumps have a rubber coating

 

if you go to a breakers yard you can use a pump from the Mercedes "same system"

 

Cool... many thanks..