On my 76 250 my gas tank is behind the seat and its capped off right after the door. everytime i go to put fuel in my tank as i release the cap it has created a vacuum against the cap. I was wondering if this is harming my fuel system. my fuel pump might be over working itself or might one day cause the tank to collapse. i was wonder what the best solution would be.

 

I don't know if this is the best solution, but I drilled a very small hole in the cap for the rear tank on my '78 F150. I thought it would extend the life of the tank and possibly the pump.

 

Buy a new cap. The vent isn't working. I wouldn't drill a cap, if you were ever to roll it over gas will run right out of the tank.

 

Mine was a new cap.

 

Some caps are vented and some are not. Some of them are marked.

 

The tank should be vented one way or another. My 77 vented through the fuel vapor cannister I believe.