On 28/01/2018 09:43, Grumps wrote:
Hi all.
This sounds a weird one to me. In the bathroom of my daughter's flat
there is a toilet, sink, and bath. The bath has mixer taps and a
flexible hose to a shower head. Cold water is mains fed. Hot water is
from a water tank at mains pressure.
Everything works, but...
When you turn the shower head on, after several seconds water starts
dripping/running into the toilet bowl. I've only tested this briefly
using the cold water to the shower head. Running the sink or bath taps
does not cause this to happen.
What's the cause?

The toilet cistern is ceramic with a dual push button thing on the top.
If I try to lift the cistern lid, I am stopped by the mechanism attached
to these buttons. How do I remove the lid?

Ta.

So, I've managed to get the lid off the cistern just by lifting the two
plungers and unscrewing the large plastic screw that is revealed.
When the shower is in operation, it is obvious that the toilet fill
valve (like this:
http://www.flushking.co.uk/adjustabl...ve-brass-shank) admits
water until it reaches the internal overflow.
The fill valve operates normally when the toilet is flushed.
I'm still at a loss to explain fully what is happening, but does this
type of fill valve rely on incoming water pressure to hold it closed?
That, as I think others have mentioned, might explain it.
Solution: replace fill valve?