Charcoal canister

I bought my F150 a few months ago. It has been driven very little the past several years (~1000 miles per year or less each of six years). I have done the basic tune-up and maintenance stuff and the truck has run great for the winter. At one point I found that the vacuum line from the intake to the charcoal canister was not hooked up at the canister. Assuming it fell of, I reinstalled it. The truck has ran several months (notably winter months) very well since then. The other day it started cutting out on me when I went to back up a hill. This got progressively worse until it would consistently cut out and even stall when I tried to go up a hill forward or backward. I checked the fuel filter and air filter to start with and changed the fuel filter just in case. Remembering that the line was off before I pulled it off again on a whim. This totally solved the problem. It seems to run fine like this. However after leaving the truck parked on a hill (rear/fuel tanks down) I noticed a fuel leak under the canister. The canister is now full of fuel. I did not notice it being full of full in the past and had no leakage in this area. The leak now is quite considerable. During the time the leak first showed up the selector was set to the rear tank (duel tank system) and both tanks were 1/2 to 3/4 full. It sat on this hill for about 2 days with ~35 degree nights and +70 degree days. Prior to the past few days I've been keeping the truck in the garage (limited temp changes and level surface). I'm assuming it is somehow drawing a suction on the tank due to thermal expansion/contraction, but this seems odd. Isn't the vapor line tied in up in the filler necks of the tanks? I can't see how fuel would be up that high. I haven't filled it with fuel in a few weeks and typically don't "top it off" too much.

Any thoughts on what this might be stemming from?