worcester bosch 230 boiller pressure loss

[spinnaker]

I'm new to the site so please bare with me.
I've got a worcester bosch 230 boiler which is losing pressure. (I know it's a bit long in the tooth but it generaly works very well!) I've checked for leaks and also ensured there is no loss from the relief valve. However when I checked the expansion vessel there was no air in it, also there was no water coming out of the schraeder valve so I assume that the diaphragm is not leaking. I repressurised the exp. vessel to 0.5 bar and a few days later the pressure had gone. Could this be due to a falulty schraeder valve? and if so can they be replaced?

 

[MANDATE]

They're the same as in car tyres, try Halfords.
When re charging the air side it needs to be 0.8bar or 1bar. Also the water pressure needs to be zero and a means of discharging water which is taking up space in the vessel reserved for air.
With vessel empty of water, but full with air, the diaphragm will be fully biased to the water side. This allows air to be compressed when cold water is heated and expands in volume. The cold water setting should also be 1bar which will hold the diaphragm to the water side of vessel, until the temperature and pressure begin to rise. The rise in pressure is around 1bar.
If the pressure gets near to 3bar which opens the PRV, it's a sign of expansion vessel problem.
Unless there is a internal leak across from mains water at high pressure to the system water, but this would open the PRV without heating being on.
You say there's no loss from the PRV, but any hot water discharging, would dry up and be misleading.
Best to secure plastic bag on discharge pipe and see what it collects.
Most often this type of problem is firstly related to the expansion vessel, and then leaves another problem because PRVs once opened seem to fail to seat again fully.
There are of course other places where leaks can take place.
Isolating the boiler from the system water and observing pressure gauge will indicate which side of the isolating valves the leak is on.