You once could lift the lid on a balky toilet and mess with the old ballcock in search of a solution. Adjusting it a bit could remedy a lot of problems. Today, they don't make toilets with the traditional ballcock and arm assembly. Still, while new toilets may seem more complicated, they are actually easier to repair. With some basic plumbing knowledge, you can repair a toilet without a ballcock.
Turn off the water supply, located behind the toilet, and open your toilet tank lid. Your running toilet may be due to a clogged seal in the fill valve. Made of plastic, the fill valve is located on the left side of your toilet. Use pliers if the valve handle is stubborn, but be careful not to break it. If necessary, spray lubricant on the handle.
Gently pry off the fill-valve seal cap with your screwdriver. Blow out any dirt in the system by placing a small cup, upside down, right above the fill valve. Turn on your water supply a couple of times and any dirt will shoot out. To prevent water from getting all over your bathroom, ask a helper to operate the shutoff valve while you hold the cup.
Replace the seal if necessary with a new one. It may be bent out of shape from dirt, calcium or other deposits. Simply place the new seal in the depression from which you removed the old one. Replace the fill-valve seal cap. Turn on your water and check for leaks.