Hi,
I've done a search through the archives, and haven't found anything, so
please forgive me if this has been asked before.

I have:
toilet cistern;
side-mount fluidmaster valve;
cistern fills from loft tank;
I've removed the pressure reducer from the valve;
I've fitter the low pressure washer;
I'm in a hard water area.

the problem is this:
the valve worked fine initially - silent and quick to refill. However,
after a few months, water started coming out of the blue air vent at the
top (which I think is there to allow air to be sucked in when there is
any sort of back-pressure). I replaced the washer to no avail. I've
tried cleaning the plastic housing with a toothbrush (there is some
light brown discolouration behind the washer) - also to no avail. I've
now resorted to installing the pressure reducer thingy. This has reduced
the flow from the top, but not stopped it. And of course now the tank
takes twice as long to refill.

Any ideas please?

I was sure it was something to do with the hard water (since it took a
few months to manifest itself), but now I don't know.

Cheers,
Tim