The main vent for most houses for the plumbing system is on the roof. This vent can be used to access the main plumbing system and can impede the movement of water through the plumbing system should it become plugged or obstructed.
Running toilets are usually caused by either a damaged chain that runs from the handle arm to the seal, or by a seal that is damaged or not closing properly allowing the water to circulate. Buy a repair kit at your local hardware store and follow the simple instructions on the kit to save an expensive repair bill.