Before you go any further check that there is no water in the ballcock float.
Roof tank ballcock.
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Make sure the water is turned off by taking off the lid of the cistern or tank and pushing down on the ballcock arm.
Ballcocks also known as float valves are found in water tanks and are used to control the flow of water into the tank.
How to change the ball valve ballcock in your cold water storage tank in the loft.
There should be no water gushing out.
On a roof tank you should find the tap on the top pipe.
One of the most common toilet repairs is adjusting or replacing the fixture s fill valve.