Dripping toilet

[Grumps]

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.

[Peter Johnson]

Or lift the buttons out and undo the screw that will be revealed. If you don't have any fingernails to get the buttons out use sellotape or similar.

[Brian Gaff]

Sounds a bit like some kind of backwards flow going on due to the mixer, but I've never had a system with the tank for hot at mains pressure myself. Brian

[Grumps]

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:

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?

[Roger Hayter]

It shouldn't leak at any pressure. But as someone said a very small leak is less likely to be a problem if you adjust the float so the water level is at least a cm (or 1/2" in old money) below the overflow when the valve closes. If it does leak at any pressure at all then it might be worth replacing. It may be possible to replace the washer/diaphragm but it the valve is such a mass of fiddly little plastic bits that it is probably better to replace the whole lot.

[John Rumm]

If its like the one in the link, then its probably worth popping the top off and cleaning any crud out from around the diaphragm.

With many like that, if you turn off the water to the valve. Fully lift the fill arm, then push down on the cap and give it a firm 1/8th turn anti-clockwise, it will unclip from its bayonet like fitting. You can then take the pin and rubber bit out, and flush some water through the valve stem to clear anything trapped under it.