Toilet cisterns that are constantly running could signal a number of different problems with your toilet. The cistern (as it is commonly called in the UK and parts of Europe) keeps the water supply for the bowl prior to the flush. A fill valve refills the toilet after a flush, while a flush valve allows the water out of the cistern. You don’t have to wait for a repairman to figure out how to repair a cistern. There are a few common problems that happen with cisterns frequently.
Check for the leak location in the cistern, if the problem is a leak, as this can give you an idea of where the problem is. If the leak is at the back of the tank, it could be the supply pipe or hose connection to the inlet or the tank bolts. If a leak is present underneath the part of the tank that connects to the toilet bowl, you may have a flush valve leak.
Loosen the water supply hose or pipe from the bottom of the cistern with a pair of pliers.
Loosen the bolts on the underside of the tank and remove it from the back of the toilet. Turn the tank upside down and loosen the nut underneath. Pull out the overflow tube and flush valve assembly after unhooking it from the flush chain. Replace with a new flush valve and tighten the retaining nut. Reconnect the handle chain.