I have a used 1989 Toyota pickup which is leaking gas out of emission system (charcoal cannister). I have traced it from the vent line to the charcoal cannister. It started when the truck would sit in the sun and tank seems to build up pressure. How the gas is escaping is beyond me. Currently the gas cap is left on loose and there is no problem (I replaced the cap and it is a vented cap). I would like to fix - would replacing the cannister help? I really can not understand how the gas is coming out of the vent line. Has anyone experienced this problem or know of solution?

I have heard of that happening, unfortunately I can't remember the cause/cure.

Does your truck have a carburetor?

http://board.marlincrawler.com/index.php?topic=9762.0
This is all I could find on the subject. Could be a clogged vacuum line, bad gas cap or bad charcoal canister. Sorry I couldn't be of more help...Good luck!

often times the charcoal canister will fill with gas becuase you overfill your tank. liquid gas is sucked up into the charcoal canister instead of vapors. when fueling stop after the pump clicks the first time. you dont you dont need to get that last 10 cents of gas pumped in.

I appreciate the input from everyone. In response to comments -The gas leaking is not due to overfilling the gas tank. It will leak if there is a quarter of a tank(in fact it is never filled). This seems to be a pressure problem - not a vacuum problem. I believe the charcoal cannister (there are two) that is leaking is tied to the fuel tank vent and the charcoal cannister is open to the atmosphere (at the bottom). The gas cap is a vented cap (which works). What I do not understand is how the gas is coming through the line. The pressure seems to result from heating (ie. sun) and there is no problem if I leave the cap loose.

Check all the vacuum lines and make sure they are correctly routed. Make sure the lines on the expansion tank have not been reversed. Make sure any check valves in the vent line are working correctly. The tank should breathe through the charcoal cannister not the cap.

I had this problem on my 1990 4x4. I needed to install a new fuel bracket to the tank, problem was, the tank was rusted and old so I wasn't able to form a solid seal between the bracket and the tank. After I purchased a new tank, I was able to bolt down the bracket properly and ended the charcoal leakage. My theory is since the tank is preasurized, some air is getting inside and pushing the fuel up front to the canister.