Get some new line, and replace the one that is leaking. If that does not work, get some RTV or the sealant shown about 3 posts back, and apply that to the leaky areas. Let the seal or RTV cure overnight, and then put fuel back in the tank and see if it still is leaking.
If it still leaks, you may need a new tank. The original factory installed fuel and vent lines were a tight fit in the holes drilled into the tank. Over time, the fuel line hardened and shrank, AND/OR the hole became enlarged due to vibration, friction of dirt, phase of the moon, and the last digit of your street address or RFD number. IOW, just because it got old suffices. Replacing the tank and line both should cure the leak, the sealant may cure the leak, or just replacing the fuel line may cure the leak.
The tank getting pressurized is NORMAL, especially if there's a bulb, and a 'two fuel line' system. Even a one-line system can get pressurized if the tank has a clogged vent, or it can get de-pressurized if fuel is drawn out, fed to the engine, and no compensating air is allowed in. Back when, fuel tanks in cars and trucks would occasionally get collapsed due to the wrong cap being installed, or a ventless cap on a tank that needed venting when the fuel was removed by the pump and no air allowed in to replace the fuel volume. They got flattened by atmospheric pressure pressing on the outer skin of a tank emptied of fuel & air.
tom