leaking joint - service valve to WC cistern

28 Nov 2009
Reaction score
United Kingdom
I have just changed the siphon on my WC. When reconnecting the cold water supply (plastic thread from bottom of cistern to screw fitting on top of a service valve) I have a slight leak!! Any suggestions???
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How tight should I turn the nut? Hand tight or spanner???
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Should there be any washers or o-rings in the fitting? I couldn't see any when I re-fitted the pipe?
usually a 1/2" washer in the fitting

if its lost or perished, they are about 8p from a b and q
I wouldn't use any PTFE on these. It as its easy to wreck the threads on some of those plastic ballvalve tails..

Its the washer on the fitting that is making the seal.
You seem concerned about how tight you should have the nut - did you by any chance over tighten it first?
It sounds like you've tried to re use the old fibre washer, fit a new one (some plumbers like to wet the washer first under a tap for a second or two). The pipe to the ballvalve inlet has to be straight otherwise the washer wont make 100% contact with the plastic thread and leak.

These washer are available in singles/tens at the merchants.

Thank you all for the respones.

I don't think I over tightend the nut - I did it up firmly by hand then about 1/4 turn with the spanner.

I didn't replace the fibre washer and I didn't check if there was already one in place when I lifted the cistern back on - it was quite heavy and awkward to refit.

I'll have a go at fitting a new washer first. I can also make sure that everything is straight when I do this. I guess I need to take the cistern off again (or at least undo the fixings and lift it up) so that I can get to the washer?
as previously stated pipe has to be pertty straight for these valves.Some times doubling up with two washers will work.
"I guess I need to take the cistern off again (or at least undo the fixings and lift it up) so that I can get to the washer?"

Hey hold your hourses a second, if the inlet pipe is has zero up/down movement then you may need to lift the ballvalve (undo nut, drain cistern etc) but there's no need to start disturbing the cistern unless theres something I'm missing?

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