Cistern overflow

My toilet cistern has started leaking thru the external overflow tube, what's the most commonest cause of this please. MBS
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In an earlier contribution to this discussion,

The *only* cause is that the water level is too high!
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The float valve isn't closing properly when it reaches the water level mark and is putting to much water into the cistern.
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Mr Blue Skye wrote:

Sometimes bits of grit get caught between the rubber washer thinugmmy and the inlet and pushing the float to the very bottom end of its travel can dislodge it.
If not, then I'll bore you with a story. Many years ago, when I was about 15, my mum's loo used to suffer badly from overflowing. I'd taken the blasted mechanism apart many times, replaced various bits, etc, but it still would occasionally not seal up properly. Then I found that a whole new float system was about 4 from Wickes...
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Scott

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Mr Blue Skye wrote:

Washer going on the inlet valve ('ball valve') - replace either washer or whole unit. You can go for fancy Torbeck/Fluidmaster etc if you want (and if it'll fit). If the original is like a brass cylinder with a brass rod with the float (ball) on and a hole for the water to come out on the underside of the cylinder then it's a piston type and shouldn't be fitted on a WC cistern so replace it anyway. A plastic diaphragm valve (with a plastic girder-like lever with a forked end) costs about as much as a pint + a packet of crisps and does the business well enough.
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