You said you hooked up a gas can and no "fuel movement". So what did the dangling fitting have to do with it at that point? Then you stated you hooked up a gas can again and this time it worked??
ALSO, I would not run tranny fluid or air thru a diaphragm pump, if that's what it is/If that's what you did. You could damage it.
ALSO: The sender, mounted at top of tank, is a combo: float, rheostat, pickup tube and outlet tube. The gas is sucked out the top thru that outlet to pump. No fitting like you show!!!!. What the heck do you have for a gas tank? The tank is behind the seat. Only fitting on bottom of tank is a drain.
You should be able to look at the old pump and see wear to tell how it works against cam and also determine if the new one is correct.
