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The metal pipe that comes out of the wall and feeds water into the cistern on my toilet is connected to the plastic cistern pipe (inlet valve pipe?) by a service valve. It is pretty much identical to this one:

http://www.toolstation.com/shop/Plumbing/Valves/Service+Valve+15mm/d20/sd2696/p44747

There is a leak in the connection between the female end of the service valve and the threaded plastic pipe to the cistern. The leak stops if I turn off the valve (so at least that's something) but starts again as soon as I turn it back on. Not a serious leak, just a drip, but it drips about a litre in 12 hours so without a big bucket under it, if I leave it on it will flood my bathroom while I'm at work.

I figured it was the washer that had gone, so I bought a pack of these:

http://www.toolstation.com/shop/Plumbing/Washers+Fixings/Fibre+Washer+38/d20/sd2703/p91174

I took the washer from the service valve to the shop when I bought them and compared them, so I got the exact same ones (or at least they look and feel identical). I have now tried every combination I can think of, and haven't been able to stop it from leaking. I have tried with a single washer, with two washers together, I have wrapped PTFE tape around the thread on the plastic pipe, I have even tried a washer wrapped in PTFE tape, but nothing seems to help.

My question is: - Is it worth replacing the whole valve? Could there be something wrong with the valve that isn't just the washer? The valve part works fine, because the leak stops when I turn it off. I did notice when I took the old washer out that there seemed to be tiny chips of what felt like concrete or something in there, not sure what that is or where it came from. - Could there be something wrong with the thread on the plastic pipe, or is this unlikely to be the issue? - Is it worth trying a rubber washer instead of the fibre one that was in there already? - Should I be tightening the valve onto the pipe really super hard, or should a few turns be enough if the washer is doing its job?

I have done a few bits of pieces of DIY before but I have not done much plumbing, so any suggestions would be welcome!

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Note my toilet/cistern is a top-loader, much like this one plumbnation.co.uk/site/… –  Matt Jan 29 at 12:51
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Considering the low price for the service valve you linked to, I'd just get a new one and try it. I'd also feel the pipe ends for any abrasion and sand them smooth with 220grit sandpaper if there are minor abrasions. Also, check to ensure the end of the pipe isn't cracked. A crack in the pipe at the end is a likely suspect for this problem. –  getterdun Jan 29 at 14:57
    
Thanks getterdun. I guess my main reason for not replacing the valve so far is that if I turn the valve off, there is no leak - if I replace the valve and it starts leaking at the other end (before the tap) then I will have to turn off the water to the whole flat :( How easy would it be to spot a cracked pipe with the naked eye? Or is there some test I could use (like wetting it and looking for bubbles etc) –  Matt Jan 29 at 15:19
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The plastic pipe is more likely the problem than the valve. The whole system is dependent on the pipe end sealing against the washer. Like getterdun suggested, be sure this is smooth. Also ensure the valve surface under the washer is clean and smooth. A neoprene washer is worth a try. You cannot tighten this type of washer too much. The fiber type can handle more pressure, but still does not need to be super tight. –  bcworkz Jan 29 at 19:47

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up vote 0 down vote accepted

Thanks for the comments getterdun and bcworkz.

I went to Toolstation and bought a new service valve and also a new inlet valve for the princely sum of about £11. As per your suggestion I checked the plastic pipe, and it looked a bit rough around the end and one of the teeth of the thread was badly worn, but I couldn't see a way to replace just the pipe so I replaced the whole thing (dismantled the cistern and everything as per a video on YouTube).

I couldn't get the old service valve off, couldn't get enough grip to turn the valve part opposite to the nut part, so I decided to leave the old one now but give it a really good wipe and test anyway with the new inlet valve and pipe. I also used a fresh fibre washer. At first when I turned the water back on it leaked quite a bit and I was slightly disheartened, but I remembered reading somewhere that fibre washers slowly expand as they get wet, so I just put a bucket under the leak and left it for an hour or so. Bingo! No more leak :)

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