My WC cistern empties itself, but not into the pan or overflow. Where does it go?

[Geoff french]

Hi All
I am a new member and this is my first post. I have been doing DIY for some 50 years, but I find this one a total mystery:

My WC (mains water) inlet keeps running, but slowly. I have replaced the valve but still it doe not shut off.

To save wasting water I turned the maintenance valve off until I could investigate further. When I returned an hour or so later the cistern was empty. No leaks found. I allowed the cistern to fill again and added some blue dye to the water. The cistern emptied itself again, and then I looked for blue water. None seen.

This blue water has disappeared, nothing in the pan, nothing in the manhole outside. The old original overflow pipe is capped off in side the cistern.

Can anyone help please.
Geoff.

 

[stevetheplumber]

is it a syphonic pan if yes water can empty straight into the soil pipe

 

[Geoff french]

Hi Steve
Thanks for your reply. The pan is an old washdown type.
Regards
Geoff.

 

[rpm]

Dry around under the rim of the pan then lay single squares of dry paper around under the rim and leave for awhile, if paper becomes wet there is you answer.

 

[Geoff french]

Thanks RPM
I have tried your suggestion of putting dry paper under the rim, but it did not solve the problem. I allowed the cistern to fill and turned the maintenance valve off and the cistern emptied after about 20 minutes, but the paper was still dry! Any more ideas?
Regards
Geoff.

 

[gassafe]

Thanks RPM
I have tried your suggestion of putting dry paper under the rim, but it did not solve the problem. I allowed the cistern to fill and turned the maintenance valve off and the cistern emptied after about 20 minutes, but the paper was still dry! Any more ideas?
Regards
Geoff.

There's not many places the water can be disappearing!

If it's not down into the pan & the overflow is capped off then maybe the valve is siphoning the water back down the pipework if you turn off at the mains & are opening a lower tap? It's a long shot tho, I had it once on a newer style fluidmaster when draining down.

 

[Geoff french]

Thanks for your reply. I will try this but don't hold out much hope. I might just fit a Fluidmaster kit.
Regards
Geoff.

 

[Phil]

Geoff can you post a picture of your setup...the toilet I mean. Maybe it is a double trap toilet.

 

[rocketmanbkk]

Are you sure it's not going down inside the pan?

 

[jonnyswamp]

Post 2 I'd go for
Other than that, disconnect the 4" outlet pipe and try again
It's got to be going somewhere, unless you've got a couple of industrial dehumidifiers nearby