Variation on the leaking cistern question

[user 28607]

Cannot find the answer to this precise situation so any advice would be very much appreciated, possibly more UK relevant as I'm not familiar with other countries specifics:
Our toilet cistern is fitted with separate fill and flush valves (both Fluidmaster now). There is no physical connection between the two valve units.
The fill valve works correctly and definitely does not overflow into the pan.
The problem is the flush valve (the third I have tried since discovering the fault):
Water very slowly seeps into the pan - at a rate that is quite difficult to see but has built up a trail of limescale from rim to water level over time.
I have read (and done) replacing seals & cleaning joints, hence why now on third valve unit. They are fitted according to manufacturers instructions, taking care not to overtighten which could distort the plastic fittings -(I'm told).
Here is where my situation is slightly different from all those I've seen posted online: the leak ONLY happens when the cistern is fitted onto the pan.
I have removed the cistern and placed it on a bench, supported above a mirror and old clear fish tank. I have then slowly filled the cistern with coloured water to various levels, up to but not over the top of the overflow tube. After an hour, there has been absolutely no leakage from the valve into the fish tank below.
As soon as the cistern is reconnected to the pan and even partially filled above the valve seal level, there is an instant leak of water into the pan.
The cistern simply sits upon the pan with the outlet end of the flush valve inside the doughnut seal (new as well) and is secured by two bolts.
Please advise if you've come across this too!
Thanks in advance.
Paul.

 

[TomFOhio]

Can you take a picture of the flush valve so we can see what you have. When you say pan is that the same as the
bowl here?

 

[user 28607]

Hi @TomFOhio , yes the pan is the bowl. The flush valve (and overflow) is the white unit, while the fill valve is the black/grey one. They are physically separate from each other and water from the fill valve cannot get into the bowl without going down the overflow tube on the flush valve. At the bottom of the valve is a flat plate with a 1.5mm/2mm thick silicone seal fitted below that is held down by the weight of water as the cistern fills.

 

[TomFOhio]

Can you get a new silicone seal for that flush valve. If that seal is bad water will leak from there into the bowl and then the
fluidmaster will fill the tank as the level goes down.

 

[user 28607]

I tried new seals before changing the flush valve (which is now the third). The original fitted when the bathroom was remodelled lasted 15 years. As I mentioned, there is no overflow issue - the leak is so slow that the level does not drop quick enough to trigger a refill in between normal use of the toilet throughout the day. We have very hard water which builds up lime easily which is how we first spotted that there is a very slow leak.