I have a closed oil central heating system. Just last year for no
apparent reason the hot press stopped getting hot. I replaced the pump
and the expansion vessel.

I also notice that the pressure at the expansion vessel is quite low -
about 0.5 bar.

I cannot see any leaks

I have blead all of the radiators

All of the radiators heat up properly - except the tank in the hot
press

Please help - Thanks

Rob