Toilet cistern leaking

I just noticed a very thin trickle of water running down the back (inside) of the loo pan. Flushing the cistern didn't stop it: as the cistern refilled it started again. The flush itself operated exactly as normal.
The flush valve is this type:
http://i246.photobucket.com/albums/gg102/BertCoules/Flushvalve.jpg
Could it be as simple as a break in the seal where the valve sits on the floor of the cistern? Or is the fault likely to be with the valve itself? I suppose it wouild be easy enough to replace the whole thing if necessary but I can't find that particular model on sale in the UK.
Many thanks.
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Bert Coules put finger to keyboard:

Did it start as soon as the cistern started refilling? What does the water level get up to? Modern WC overflows go into the pan.
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Is it one of those bloody awful floating valve things, button on top of the cistern? Mine did the same thing after 4 years. We were actually insured for it :-)
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On 05/06/2013 10:44, Bert Coules wrote:

The one you picture looks like a syphonic design, so if its letting by once the cistern refills, it suggests that the water level in the cistern (set by adjustment of the float valve) is marginally too high and its overflowing down the pan.
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Thanks for all the replies. The lever is at the side of the cistern. If I'm right about where the overflow is, the the water level when full is nowhere near that high: it's just below the 6ltr mark, which I assume is the suggested maximum.
The leak doesn't start immediately the cistern begins to refill: it only happens when the fill has ended. Perhaps I'm wrong about the overflow. Here's a picture:
http://i246.photobucket.com/albums/gg102/BertCoules/Cisternoverflow.jpg
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On 05/06/2013 15:34, Bert Coules wrote:

That isn't the overflow Bert. It would be part of the thing with the green top.
The fill valve isn't adjusted properly, or it's FUBAR.
Turn the rod at the other end of the arm clockwise to lower the float & that should sort it.
--
Dave - The Medway Handyman www.medwayhandyman.co.uk

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On Wed, 05 Jun 2013 17:22:20 +0100, The Medway Handyman wrote:

If it carries on filling, the fill valve may not be shutting off properly. May not be knackered, that type are notorious for leaking when there's dirt or grit in them.
You clean it by: 1) Shut off the water inlet 2) Unclip the (rather fragile) white rod and float, noting the point that it's clipped onto the rod. 3) Unscrew the blue ring and remove filter and diaphragm. 4) Clean everything and make sure the tiny holes are clear (canned air helps). 5) Reassemble, making sure the little needle on the main part goes through the small plastic doughnut in the diaphragm. 5) Reattach the float, water on and test.
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Bob Eager wrote:

Thanks very much for that, Bob. Beautifully detailed.
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On Wed, 05 Jun 2013 21:00:15 +0100, Bert Coules wrote:

I've done it too many times recently! When they demolished the brothel next door and built a house, they kept faffing around with the shared water pipe. At least it's not shared any more.
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On 05/06/2013 15:34, Bert Coules wrote:

No, that's just the back of the filling spout on the inlet valve.
The overflow is either a function of a separate 22mm PVC pipe coming out of the side (typically) or sometimes bottom for the cistern, or by the water level simply reaching the height of the outlet pipe on the syphon. Once that happens, water will naturally pour over the edge of the syphon.
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That's not the overflow - it's probably an air inlet to the fill valve to prevent any chance of syphoning the water back into the water main if the supply pressure fails.
If there isn't a separate overflow outlet pipe, then it's designed to spill over the main syphon into the WC (which is now the norm with new cisterns).
The fault is likely one of two things; the valve not working anymore, or the fill level set too high.
Try lowering the fill level. You do this by turning that thin shaft with the very course spiral thread on it, so the white cylindrical float is lower. If the water comes up over the top of the white cylindrical float when it's filling, the fill valve is knackered. The water should lift that float, and cut off the filling, with the float still rmaining partially above the water surface. If the float is too high, the water will over-top the flush syphon before it cuts off, so it will continue filling forever.
--
Andrew Gabriel
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Thanks to everyone for the new replies. Several people advised lowering the fill level: I did that, flushed the cistern, and checked the leak. It didn't return, even when the cistern had completely refilled to its new level.
I left it an hour or so and looked again: the water was higher than the original level, and actually above the top of the float. Andrew Gabriel said:

Well, not while it was filling, but afterwards. So it seems that the cut-off valve isn't cutting off. I'll see if I can disassemble it and take a look: the water here is extremely hard and things might simply be furred up.
If I can't fix it and have to replace the valve, two questions:
Do I have to use the same model as the existing one?
Do I have to completely remove the cistern? I've only had a quick look but I can't see any obvious external fixing for the valve assembly. Or is it secured purely from inside the cistern?
Many thanks.
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On 05/06/2013 20:04, Bert Coules wrote:

Something similar, but it does not need to be identical.

Look under the cistern, there will probably be a pipe running up to the bottom right hand side with a tap connector on it. Shut the water off (using the service valve if there is one (or using this an an opportunity to fit one!), disconnect the tap connector
Now flush!
...then undo the plastic locking nut at the base of the threaded bit that sticks out. Lift the whole valve out (have a container ready to catch the bit of water left in the cistern after the flush.
New one drops in on its rubber washer - I normally stick some silicone grease on both sides of it to help it seal.
Stick the lock nut back, and then reattach the tap connector. Note that they are very easy to cross thread - so make sure you get it on dead straight.
--
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John Rumm wrote:

Thanks very much. You don't mention unfastening the central part of the valve - the bit which leads to the connection to the pan. Is this then not attached in any way? If it isn't, how is the seal maintained? There looks to be a rubber washer on this part too. It was that central connection that I was talking about when I said I couldn't see any obvious fixing on the underside of the cistern.
I'll take a proper look at the whole assembly in the morning. There is a stop valve in the supply pipe so at least I don't have to fit one of those.
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I wrote:

Ah. I hadn't realised that the syphon mechanism is separate from the inlet valve, I thought it was all one assembly. Sorry about that.
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On 06/06/2013 00:49, Bert Coules wrote:

Different valve and different purpose...
You have the flush valve / syphon[1]. This is the thing with the green top in your photo. That is responsible for flushing the loo. I am assuming that this is actually working correctly - and the only reason you are getting water flow through this when not expected, is because the inlet valve is not shutting off completely when the cistern is "full". Hence the level keeps rising, and eventually the level gets to the gets to the top of the inverted U pipe in the syphon. At that point is falls over the edge, down the outlet and into the pad. However since it is only a trickle and does not fill the syphon pipe it does not trigger a full flush.
Secondly you have the inlet / float valve - the thing with the blue locknut on your photo. This senses the water level and allows the cistern to refill. If its working correctly it will stop it filling before it overflows. My guess is that yours sort of stops - but not totally. So after a delay the level rises to high and overflows. The overflow on your loo being designed to work into the pan, rather than out of a supplementary pipe.

[1] These things are basically like an inverted U shaped pipe in design, with a little flapper valve on the inlet side that can be used to "drag" some water up the pipe. This tips the level over the top of the U and it then starts syphoning. It keeps going until the water level in the cistern falls below the bottom of the inlet of the syphon, and that lets air into it which breaks the syphonic action and it stops. These have the advantage that even of the flap valve gets leaky - it just makes it harder to start the flush going. The valve has no involvement in stopping the flow of water after the flush, the emptying of the syphon takes care of that. (there are some modern light touch valves that don't use a syphon, and do require the flap valve to seal well to stop the water - but yours is not one of those)
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John.
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I just discovered something I hadn't noticed before. John, you said:

There's rather more than "a bit" of water left. I'd say the water level in the cistern immediately after the flush is about a quarter of what it is when the cistern is full.
Doesn't that suggest that there's something more fundamentally amiss with the valve than just the inlet not sealing properly?
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On 06/06/13 01:18, Bert Coules wrote:

I used a bit of pl;asticd pipe to siphon the water into a bucket. Very little spill.
I also repaired the valve and put the while thing back as it was.

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On 06/06/2013 09:12, The Natural Philosopher wrote:

Even easier to use a "wet and dry vac", if you have one.
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On 06/06/2013 01:18, Bert Coules wrote:

The amount left varies with the design of syphon and cistern... tis why I said container rather than cloth for catching it.
(I sometimes suck it dry with a wet'n'dry vac first)
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