The assumption is it's an all plastic valve and float unit inside the cistern.I would say the valve washer needs replacing.
Float arm adjustment.
At the valve end of the float arm is a small plastic bolt with locknut with a slot in the end. Flush the loo and then turn the bolt two full turns clockwise [towards you]if the valve is on the right of the cistern or two full turns away if the valve is on the left of the cistern.
Replacing/checking the float.
The red float-ball can develop a crack, or leak, and if it gets water inside it, will not work efficiently as a float. If you hold the lever up, the ball can be unscrewed to inspect it, and ensure that no water is inside it. Most DIY and plumbing shops will stock these, at around £1.50.
To carry out the following you have to turn off the water to the cistern.
The diaphragm that is inside the valve, can develop a small hole, and leak. These can be replaced, but there are 2 or 3 different types and sizes. Scale or debris can build up on the diaphragm, but either way, you have to isolate the water inlet pipe, and unscrew the plastic ring-nut holding it and as you gently remove the nut valve head and the rubber diaphragm note which way round the diaphragm faces. Clean it up and inspect the diaphragm for pin-holes. Re-assemble, and test if water still fills up to far adjust the float arm by the small nylon bolt at the valve end of the arm. To shut the water off sooner turn the nylon bolt in toward the valve.
To let more water IN turn the nylon bolt towards the float ie: reduce the distance of the valve end of the bolt so it shuts off later letting more water in. If you've got a digi camera take a picture of the valve before dismantling and when dismantled laying the nut pressure head and diaphragm in a line in order of assembly then take a picy you won't forget then how to re-assemble the valve. That goes for anythng your not sure of. I did with my alarm control box in the house when I was working on it.
Note that older float valves will have a brass cylinder inside with a flat rubber disc in one end. You usually have to remove the brass cotter-pin and float arm, to get at this, it just slides out.The cylinder unscrews into 2 parts, releasing the rubber disc. Just replace and refit. If you have had to turn off ALL the cold water to do this, now is a good time to fit an isolating or service valve.
Rob
Rob, November 2005
