A misaligned or old toilet ball valve may be the culprit in a toilet that constantly runs. This ball-shaped seal stops water from running into the toilet when it is lowered and allows water to flow for flushing when opened. There is more than one cause for a leaky ball valve, so check thoroughly before buying replacement parts.
Check the valve
Remove the lid from the toilet cistern. Examine the ball valve and determine if it is seating correctly. Examine the wire that is attached to the guide arm on the overflow pipe.
Turn the water off using the valve behind the toilet or, if you can't find this, at the stop-cock where the water supply enters your home. Then, straighten the wire if it is bent and adjust the placement on the guide arm, if necessary. The wire should be placed on the guide arm where it is lifted high enough to open the valve during a flush and lowered low enough to seal when finished.
Turn the water back on and test for leaks. If the valve still leaks, it is time to replace the assembly.
Replace the valve
Turn the water off. Remove the old ball valve assembly from the guide arm on the overflow pipe.
Slide the lower lift wire through the upper lift wire and its guide. Attach the upper lift wire to the guide arm.
Thread the ball valve on to the lower lift wire. It should be snug, but not over-tightened.
Align the ball over the drain, making sure the wire is vertical. Use the flush lever or button a couple of times to be sure everything fits properly.
Turn the water back on. Check for leaks, and make any necessary adjustments. If none are needed, the toilet is ready for use.