UKH

Our toilet cistern has developed a problem.

After flushing it fills a bit, stops filling, fills a bit, stops, fills a bit, stops, fills a bit..you get the idea.

On inspection it has one of those plastic fill valve assemblies, (what is wrong with a good old ball cock?).

The float sits in a plastic cup, which I presume is just to hold it in line as it floats up and the conecting rod pushes up to shut off the fill valve.

Whats happening is the 'cup' fills with water that runs out of the fill valve, the float..er..floats up and shuts off the fill valve. The water then runs out of the bottom of the cup, dropping the float and the whole cycle starts again, (and again, and again...) until the cistern fills to the set level.

There appears to be some sort of plastic valve in the bottom of the cup for some reason that I can't comprehend (other than to break so you buy a new fill valve).

Can I just remove it?

Hmmm

Possible air lock in cold water feed to toilet. Try running all of the cold taps in house for 2 minutes then see if it improves.

Thanks Lee

Cold water feed is fine, if I hold the float down manually it runs well. It's the float valve cycling up and down thats cutting off the water flow.

Whoever invented those stupid things they've definitely wasted a lot more water than they've saved because they never work and are impossible to fix

Presumeably the water flows out of the cup before it has filled up.  I suggest you need to slow the inflow to allow the outflow to do its job.  You pipe should have an isolation valve closed with a screw driver.  Just reduce it until the inflow and outflow speeds in the cup are balanced.  Usually these valves come with a choice of inserts for high/low pressure systems.  Presumeably the numpty who fitted it didn't know what it was or just fitted the wrong one.

Alternatively I assume the float is only supposed to float up when the cistern is full, and the problem is too slow an outflow from the cup.  Is it blocked/limescaled up?

Must be. Seems a pretty stupid design to have a restricted outflow from the cup which serves no purpose (other than get limescaled up so you have to replace it)

Looking at the valve there's no way to strip it down in situ to have a look, and it's constructed of what appears to be the thinnest, brittlest plastic ever invented.

Looks like live with or replace it.