There are several things that can cause your cistern to stop working as it should. Here are a couple of the most common cistern problems, along with ways to fix them.
Corrosion in the cistern
If the cistern is showing signs of rust drain the water and remove all traces of rust with a wire brush or steel wool. Fill any pit marks with epoxy resin. When the filler has set, apply two or three coats of non-tainting bituminous paint. If the cistern is beyond repair, replace it immediately.
If your cistern overflows the first thing you you need to check is that the float isn’t a leaking. Unscrew it from the arm and shake it, if you hear water inside it will need to be replaced. If water gets inside a float it makes it too heavy to rise to the correct level to shut off the supply. If the float is still sounds, without leaks, it may just need to be adjusted. There is a small screw at the end of the float arm which you can use to adjust the level of the float. If the overflow persists after adjustment, it is possible the valve needs a new washer.
To replace a washer you first need to shut off the water supply, flush any water out of the system and are then look under the valve for a split pin. Remove this and, using a screwdriver, slide the piston out of the valve body. You can now unscrew the end cap of the piston and remove the washer. Making sure you have a replacement washer of the same size, insert it into place. Reassemble the valve, slide the piston back in and reconnect it to the float arm. Switch the water supply back on and then adjust the arm accordingly.