porcelin leaking?

[Jennie]

Hi all,
I've been out to fix a leaking toilet this week. It has a low-level cistern, hung on the wall.
The customer reported a very very slow leak, coming through from the flush valve under the cistern, continuing when the toilet wasn't used for any length of time.
It has a dudley turboboost inside. I've changed the washer, and put the original turboboost back in place. I didn't use sealant or plumbers mait, but - as usually works fine for me - made sure the porcelein was totally dry and clean.
Two days later, the leak is reportedly continuing.
On the underneath of the cistern, the area around the flush pipe was damp and discoloured.
I'm wondering if there is a hairline crack in the cistern, or a burr, etc, where the washer sits.
I'm tempted to replace the cistern itself.
What would you all do?
Thanks for your advice,
Jennie

 

[rpm]

Any chance it`s the water coming out of the fill valve hitting the lid and running down the back of the cistern or it`s one of those cisterns with a stupid U shaped bit cut out of the back up high near the lid and water is leaking out there or it`s purely condensation and it`s running down the back?

 

[Phil]

I hate myself but this if why I assemble with a smear of Clearfix as you can use it in wetness. Stops these annoying calls to say it leaking can you come?

 

[Riley]

I've had this before and it turned out to be a hairline crack

 

[NickDavies]

I had one of these once the thing was covered in sealant, at the flush pipe outlet, absolutely caked in silicone, the cistern fixing screws had been overtightened when it was installed and it was the tiniest hairline crack.........good call to replace it