1980 204d will not pull fuel

hello,
our (new) 1980 240d 4sp manual won't pull fuel from a can of diesel purge. i have done this procedure with 2 other 300's. fuel lines flow freely from tank filter to spin on (hand primer dosen't leak, but barely moves fuel--could this be a potential plug point?), and filters changed. The filter in the tank was badly plugged with algae, now free. Now there is no fuel returning to the tank thru the "cigar" hose. i am used to seeing it fill a plastic bottle quite quickly if bleeding the system. in fact, the car quit when i tried to run a can of diesel purge. i then removed the spin on filter, and noticed the fuel had dropped almost 1/2 way inside. This happened twice, on different occasions. There also mysteriously appeared in the purge bottle, a slug of algae! How did this make it past both filters?? so the car pulled air, and quit. Previously, we had noticed rapid powerloss up hill during a heat wave (100F), stalling once. While traveling, i had to turn the idle up to keep it running. These symptoms have mysteriously gotten a little better with cooler weather, and more driving. Seems worse when tank is low. it "seems" like the car is relying on head pressure from the tank to keep running. is this possible? Car also has possible injector "nailing" noises at 50mph. 320,000+ miles on car. car starts instantly when cold, has a shudder at idle that gets worse when i turn steering wheel in park, but otherwise runs well, and gets over 30mpg. But why isn't the pump returning any fuel to the tank??
thanks for your advise!