Gary, your injection pump will work just fine when gravity fed. Hopefully you ran the fuel through the secondary filter before the injection pump. Could be an expensive experiment if you didn't.

How's the fuel looking (from the tank and secondary filter)?
Getting fuel out of the tank by gravity?
Getting fuel from the tank to the fuel pump (the gurgling sound is a big clue)?
Fuel pump working? Crack the vent plug at the top of the secondary filter when a friend cranks the engine, and have lots of rags handy. Should be no bubbles after a couple seconds. There's no such thing as a perfectly tight system - if there's a constriction somewhere (crud) then the system is going to suck air from a fitting or a "leaky" hand pump - the path of least resistance rule. Or more crud will get loosened up.
Working from the source (tank) will save time and frustration.
Umm... ...you do have fuel in the tank? Crud in the tank can make the fuel gauge sender "flakey". An easy clean as long as you're in there.

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daBenz - 1970 220D