A fuel cell has a non-vented cap. Without a vent, the cell would get really big when the temperature increased!! And when you sucked fuel out of the cell, you would reach a point where you couldnt suck any more....so you need a vent. The vent usually exits the top of the tank. If you had a full tank and parked on a side hill or went around a corner fast, fuel can come out of the vent. If you rolled over, fuel would come out of the vent. To prevent this, a ball check valve is put into the vent line. This doesnt prevent the sloshing out of fuel so you DO need a line on the vent fitting. Run it upwards as far as you can....we have a 24 inch rise on ours and it doesnt let fuel exit. If you like, you can put one of the little K&N filters on the end of the vent line...they are about 1 1/2 inches in diameter. This keeps crud from accumulating in the end of the vent line.