I'm working on a geberit kappa concealed cistern which has been overflowing into the pan. On inspection it was clear that it was a gasket issue rather than a filler one as it was nowhere near the overflow point, so as it was about 10 years old I renewed the siphon in its entirety. Once fitted it was still leaking into the pan. tried refitting it, same result. the gasket is obviously new and appears to be seating ok but still the cistern leaks into the pan. the basket into which it fits looks ok also, (although I havent renewed it) so i cant understand why its leaking. there is pretty much zero access apart from the small aperture provided and i dont want to start knocking tiles off to access the cistern itself if there is something i havent tried. does anyone have any idea what else i could try? many thanks.

Not that particular cistern, but I had the same problem a while back with a Grohe concealed cistern. It was letting by the tiniest trickle of water into the pan constantly. It turned out that the base of the flush valve, which had little lip on it that made it look like the obvious place for the sealing washer to sit, was actually not the place for the washer to sit at all. I pushed the washer further up the valve (which, to be fair, was illustrated, albeit in a microscopically small picture, in the instructions that I didn't look at properly) and it was then fine. I wasted a lot of time on it though.

Other than something like that, the only other likely cause that I can think of is limescale or grit around the hole at the bottom of the cistern where the valve sits. I usually give them a bit of a clean with the green, scratchy, side of a kitchen scourer/sponge if they look manky. They are a pain to work on if you have big hands though. I'm not a fan of concealed cisterns for this reason.

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Haste is the enemy of quality.

I squeezed a paw in and have taken it all to bits again and given everything a good clean out seal wise as best I can. maybe its a bit better as only a slight weep now. I fear I'm going to have to knock a couple of tiles off to get a better access and fix it properly. I hate concealed cisterns without proper service panel access.

thanks for your reply.