Use a magnet to locate the blockage - It should NOT stick to copper - If it does and you loacte it, this is the reason for the Pumping Over !!
Obviously a history of Bloakces so recommend a Clean & P/Work Re-config !
Good Luck.........
Think you need a new Div Valve section and if Exp Vessel gone 90% of the time it is the Core Shraeder and NOT The Vessel itself !!
Either way an RGI is Required
Make sure all 3 stats are as you said.......
1) Boiler stat set to Max 2) Room-Stat up Hi ? 3) Cyl Stat above 60deg ?
Could be a few things - Pump, Div Valve or Gas Valve
Are pipes to CHB Hot or cold - or one hot one cold ?
Good Luck
Also, Personally I would replace the schrader Valve core within the epansion vessel - this would be one cause of the PRV Leaking.
If no further leaks are evident on the CHB/System
I'm new on here - Where are they ? Am i doing something wrong ?
RULES READ - If it is the Gas Valve you need an RGI Fitter to repair - My advice is for diagnosis ONLY !