Apart from a leak caused by a split or hole in a cistern, the most common problems in cisterns are caused by faulty valves. The float valve in the storage cistern in the loft or roof space operates in the same way as the one in a WC cistern. Most modern WCs are washed down with a direct flushing cistern. Water is supplied to and fills the empty cistern through the action of the float or ball attached to the arm. As the water rises, the ball floats up and eventually closes the valve to shut off the supply.
In a direct action WC cistern, however, there is a second valve called the flap valve. This is hidden from view inside the siphon chamber. Take off the cistern lid and you’ll see how the flush handle is connected by a wire link that goes into the siphon chamber. When you flush, the lever lifts a perforated metal plate at the bottom of the chamber. As the perforated plate rises, the holes in it are sealed by a plastic diaphragm called the flap valve so that the plate can push the water up over the ‘U’ in the chamber and down the flush pipe.
Faulty Flap Valves
A sign of a faulty flap valve is when the WC won’t flush first time – although you should check first that it isn’t the lever that is the problem. Shut off the water supply, or tie the float arm to a batten laid across the cistern, and flush. Under the cistern you’ll find a large nut holding the flush pipe to the cistern. Use a wrench and unscrew this. Remove the retaining nut clamping the siphon to the base of the cistern. A little water will run out. Inside the cistern, disconnect the
flushing arm and ease the siphon out. Lift the plastic diaphragm off the metal plate and replace with a new one of the same size. Reassemble the flushing mechanism and then re-attach the flush pipe underneath the cistern and restore the water supply.
The optimum level of water in a WC cistern is about 25mm (1 in) below the outlet of the overflow pipe. In an old WC, the arm on the float valve may be made of metal: bend it downwards a little to reduce the water level, or straighten it to allow more water to flow in. In a modern WC, the float arm is probably made of rigid plastic: in this case it is likely that the valve is not fitted with a washer, but a sealing diaphragm with an adjustable screw to regulate the amount of water entering. Release the lock nut and turn the screw towards the valve to reduce the water level, and away from the valve to increase it.
Corrosion in the cistern: If the cistern is showing signs of rust, drain the water and remove every trace of rust with a wire brush or abrasive paper. Fill any pit marks with epoxy resin. When the filler has set apply two or three coats of non-tainting bituminous paint. If a cistern is beyond repair, replace it immediately.