I just changed both fuel pumps in my 83 wagon recently(like three weeks ago).
The toughest part was removing and reinstalling the filler tube. There is a
collar on the left side of the back that you reach by angling your arm between
the body and the gas tank. It took a 10mm wrench.
I took out the tank and was glad I did since it was very easy and five bolts.
The two fuel hoses on top of the tank I seperated from the tank inside the car.
If the hose was stuck I would have, in a difficult case, try to work soapy
water between the hose and metal tube.
Disconnect the battery of course.
The electrical connection, I disconnected at the connector on the body and let
it flop with the tank when I took it out. I don't think it will seperate from
the tank access when changing the intank fuel pump.
I cleaned up the tank access really well using WD 4o and then tapped it open(
it only goes one way) with a solid brass piece of metal. It came off with
surprisingly easy taps. The inserted fuel gauge and fuel pump had to be
manuerved carefully which is one reason I was glad to remove the tank.
Ok, if there was a vaccum I would think that removing the gas cap would solve
that problem. You could also remove the hose at the main fuel pump. Mine has a
screw clamp holding it on,
It seems a daunting task but the only weary part was taking and reinstalling
the filler hose collar.
Rojo