this is because if you have black dirt particles in the water, if this water ever passed the valve seat that will stop it from sealing.
By the way don't use a pressure valve to release pressure or it won't seal properly afterwards. If you do you'll have to take it apart and clean it.
MArcus
February 2013
when u fitted the new valve what pressure did u fill the boiler to and what caused it to leak again. the prv will open up around 3bar as a safety precaution. so the pressure must have risen or it was opened manually by turning the valve. but once it opens and has not closed properly it will keep leaking.
kash
February 2013
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
