Ball valve constantly running — cistern or loft tank

A ball valve (or ballcock) that never fully shuts off is wasting water and often causing the constant sound of running water. In a toilet cistern this shows as water trickling through the overflow pipe (usually a pipe going outside, or water constantly running into the pan). In a loft tank, water drips from the overflow at eaves level.

Isolate the water supply: for a toilet cistern, turn the isolation valve on the supply pipe (a slot-head screw in the pipe — turn 90° to close). For a loft tank, there is usually an isolation valve on the rising main in the loft.

Flush the toilet or open a tap to empty the cistern, then examine the ball valve. Modern cisterns use a Torbeck or diaphragm valve (a compact unit with a float on the side) rather than the traditional long-arm Portsmouth valve.

If it is a diaphragm type: unscrew the front cap, remove the plunger, and inspect the small rubber diaphragm. This is usually the culprit — a split or hardened diaphragm lets water continuously past. Replacement diaphragms cost £3–£5 and push directly back into the valve body.

If it is an old Portsmouth-type valve (brass body, long float arm): remove the back nut, pull out the piston, and replace the rubber washer at the tip. If the valve body is heavily corroded, a full replacement is better value (£8–£20).

After reassembling, restore the supply and check the shut-off level — the water should stop filling approximately 25mm below the overflow outlet. Adjust the float arm (or float height on modern valves) to achieve this level.