Boiler on the blink, help needed :)

[sinto607276]

You shouldn't have to do anything with the expansion vessle as long as its ok, its there so when the water expands on heating it has somewhere to expand into.

I put some air in it. It shouldn't be full of air then?

If the diaphram is damaged then it won't work and when the water expands the only place it can go is out of the PRV, then when it cools there's not enough pressure for the boiler to work?
that seems to be what's happening
You should be able to pressurise the system in a few seconds if the system hasn't been drained by opening the valves on the filling loop there may be two one ate each side if the loop,(as its should be removed to prevent contamination but never is).I put the boiler on and it pressured after a while, but don't know if it'll stay up in pressure, looks like I'll be looking at a new diaphram then?
Oh that's way past my diy skillset