1994 Volvo 850 Sedan:
Whenever I fill the tank to full, gasoline has tends to seep from the top over the side of the tank. When the tank goes down to a certain level, probably around 3/4, it no longer leaks. Needless to say, I don't fill it up often because of this. Any suggestions?...

 

Have you had it to the dealer lately for this problem?
They are having a recall for a gas tank leak.
Call the dealer with your VIN# and have them check and see if your car is under the recall.

If not then it is most likely a cracked hose on the top of the tank.

There is 2 reasons a car will do that 1 is a cracked tank 2 a cracked hose.

 

I did bring it into the dealer for the recall notice and they found no cracks or leaks around the heat shield bolts, which was the reason for the recall. Volvo wants to charge $150 to drop the tank. The parts manager at Volvo sad that it might be the "O" rings in the fuel sending unit, which I think I could get to from underneath the rear seatback. There is a metal plate there. Is that a difficult and dangerous job and should be left to the professionals? How would I know if the rings are leaking. Again, it only leaks on a full tank. As always, thanks TECH!

 

Yes removing the cover you will be able to see the sending units.
You can also use a mirror and a flash light and try to see the rubber hoses and see if they are cracked.

If worse comes to worse you can drop the tank as long as you have jack stands and a floor jack.

Once the covers are off you can go to the gas station and fill it up and watch for the gas to leak.
Does it come from the drivers side(LS) or the passenger side(RS)?

 

It appears to only flow over the passenger-side (RS) of the tank, again only when the tank is full. It doesn't leak when I'm filling the tank. I did call the Service manager at my local Volvo dealer and she said that if the seals on the sending unit were bad, the whole sending unit would need to be replaced ($425)? But the parts manager at the same dealership told me he could special order the "O" rings? Which one is telling me the truth?

 

You can replace the O-ring for the sending unit.
You will have to remove the sending unit to replace it though.

Remove the covers and make sure the tank is dry then fill it up and watch where it is getting wet just after you fill it up.