fluidmaster fill valve in wc affected by bath taps

[Terry7556]

ideal standard w.c. close coupled in a house between 5-10 years old. the fluidmaster 45 (grey top) works fine ,fills up quietly to the level required when flushed
the bath taps are 2 feet away and when the hot and or cold tap is turned on the wc overflows. when the taps are turned off the wc continues to overflow untill flushed
i have a new fliudmaster fill valve to replace it but cannot figure out what is happening.is it pressure greater at the bath taps than the 15mm wc supply and fill valve can cope with?
it cant be the diaphragm at the top of the fill valve or why would it work in normal use
any ideas please as to why this is occuring

 

[HITGAS]

I would change the fill valve. They do work on back pressure to stop filling. When you open the taps the pressure is less to the fill valve. When the pressure returns to normal the valve is letting by. Sounds like a intimittent case of letting by washer. This is just a theory.

 

[Terry7556]

hi hitgas welcome to the site, your theory was correct.
a very nice young lady from fluidmaster at bristol talked me through it, saying the diaphragm at the top was worn and could,nt cope with the pressure difference
she also told me about a neat trick to just change the top of the new one for the old one.3/4 of the way down from the top of the valve is a sealing washer, pull it up an inch and the top part releases from the stem, swopped them over, all good, no draining

 

[Charnwood]

3/4 of the way down from the top of the valve is a sealing washer, pull it up an inch and the top part releases from the stem,

Normally used for adjusting the height of the valve.

 

[Terry7556]

hi charnwood
i knew about the adjustment but it never occured to me to separate the parts untill the lass suggested it. between here and ultimate handyman i got over a hundred hits on this but no one offered advice untill after the job was done. today yourself and hitgas have answered, i was surprised by that but thats the way it goes sometimes.
i thought that you would be one of the guys to answer because of your knowledge of kit in general, ive always thought your replies have been really helpful

 

[Charnwood]

Knowledge of kit, yes.
Fitting it and fixing it, now that's another matter.