How Can I Find What is Causing my Toilet to Leak?
If a faulty inlet valve is causing the problem, then water will pass constantly into the cistern from the supply hose (usually a braided stainless steel flexible hose). The level of the cistern water will rise until the excess flows into the overflow tube. This tube is connected to the flush valve and bypasses the seal to flow down into your toilet bowl. To diagnose this fault, look into the cistern to see if the level is so high that excess water is running into the overflow pipe.
If the flush valve is the problem, the water in the cistern will leak through the valve seal and run down into the bowl. The outflow of water through the leaky seal will lower the level of water in the cistern, until it causes the inlet valve to open and allow in more water to raise the water level. This cycle of leaking out and then filling back up, will continue until the leaky valve is replaced. The common way to diagnose this problem is when the house is quiet (particularly at night), to listen for the stop-start sound of the inlet valve re-filling an apparently full cistern.