Gerry, I dont think the setup on the diesel is the same because the diesel
would not need the evaporative venting that the  gas tank would( those two
grommets on top with the lines going to them. Im pretty sure that if you plug
those lines and pressurize the tank with the guage on the feed line you will
get an acurrate read.As I recall the return line just dumps into the top of
the tank and the vent lines end at the top as well.When you pressurize the
tank you could squirt soap solution around the vent and return lines to see
if you have a crack,otherwise I think it would be air tight and you could
eliminate that as a source. I believe the feed line inside the tank is a
poly/butyl or PVC black line....Maybe it has split? I think you might be able
to reach it with the sender out with a 3 finger claw tool I used to grab
screws that fall in inaccessible places.I have seen where volvos and bmw
pumps make a loud sound when the primary pump(boost pump)in the tank quits.It
sounds like air in the system, but it will still run the engine, because the
main pump will still produce pressure sufficient for injection.It seems that
if your pressure is there and the vehicle doesnt misfire, that there is no
compressible air in the line.The air would cause a misfire and corrupted
spray pattern on the injector.How about splicing a temporary clear pvc 5/16"
plastic line( the hard stuff you can get in the plg.dept at Home Depot) in
the feed before and after the pump?Say 2" clamped on each end with miniclamps
in series with the feed  on each side of the pump.Then you could see if you
truly have air in the line, and if it is coming in before or after the pump.
Then you will know if it is air(you see it) and maybe rule out the tank
altogether. II think I would give this a try....its cheap and the only
downfall is a little gas spillage here and there as you substitute hoses.Let
me know.

Ed Berroth
84 Westy Wolf
87 Stinkcrow