[measuretwice]

Being that I just replaced the fuel pump in my 93 duster (which is basically a sundance) I can tell you there is a 'boom' arm that has a filter pad that sits at the bottom of the tank. The pump sucks up fuel through this into a canister that holds fuel in the pump with a volume of about a pint. The top of this cylinder is at about the 3/4 tank height, and fuel can come out the top, or air from in the tank can go in the top. So, it sounds like the boom arm with the filter pad is clogged, and fuel (when the tank is 3/4 full) is being sucked in the top of the pumps canister. When you go below that level in your tank, air is being sucked in. The pump is $160.00 at autozone.