leaking cistern/flushpipe connection

4 Jun 2008
Reaction score
United Kingdom
Any help appreciated please. A slow leak appeared beneath the WC cistern last week. It's a low-level 2 gallon cistern with flushpipe, and the leak is coming from between the base of the cistern and the top of the siphon backnut (definitely not condensation). I replaced the flushpipe and siphon, using the appropriate siphon washer on the inside of the cistern, but the leak persists.

I've now made/remade the joint four times, carefully centralising the siphon/flushpipe in the cistern outlet each time, but I cannot resolve the problem. The leak is extremely small but VERY irritating.

The last bit of significant information: there's a very fine hairline crack on the inside bottom surface of the cistern (about 5" long) which runs into and all the way through the cistern outlet from top to bottom, but is NOT visible on the underside of the cistern.

Though clearly visible, I would definitely describe the crack as hairline. Could this really be the source of the problem, and the reason I'm having so much difficulty repairing it?

Thanks in anticipation.
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i would say the crack is the problem as changing syphons they useally seal first time.
Thanks Seco. I had a sinking feeling that the crack was going to be the problem, however insignificant it looks.

Anyone know a good place to pick up a cheap 2 gallon cistern in South London (side entry inlet and overflow)? I'm not changing the pan so I believe this doesn't contravene the water regs (i.e not a NEW installation).

Thanks again
long way to come from liverpool to s london for a cistern

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It appears that in my haste to register and post, I "mis-clicked" with regards to my location. This is similar to mis-speaking - ask Hilary Clinton!

Help re tracking down a 2 gallon cistern would still be welcome!
do you have any local builders merchants as they useally have what they call builders sets and you can buy pans, cisterns, basins seperate rather than a complete set.
Oh, and by the way.... are the outlets on cisterns a standard size? So if I compare the (flush) outlet diameter of my old 9 litre cistern with a modern dual-flush jobby (if you'll pardon the expression), should it be the same?

What I'm getting at - in a roundabout way - is: have siphon outlet diameters changed as cistern capacities have been reduced?
if you compare it with a dual flush you'll have to change the pan as well
Sorry, I didn't make my question very clear in that last post. What I was getting at was, could the cause of the leak be the fact that I'm using a modern siphon (Delchem 9.5") with an old cistern, and the outlet sizes are incompatible? - though I have to say, the siphon flange and washer do appear to seal the cistern outlet adequately. I was just wondering if cistern outlet sizes or siphon diameters have changed during the last decade or so.
9/10 are standard i have seen different sizes but you would know as you syphon wouldn't fit the hole

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