Vac breaker valve is leaking. It’s either scaled up and cannot seal correctly, or the seal itself is worn.
It is located on the service boiler here, it is the lower part, can remove it with a large adjustable spanner.
Paolo recommended dropping it into some descaler solution but that didn’t fully solve my similar issues.
I ended up disassembling mine for a proper clean and replacing the seal, this helped but hadn’t fully solved the problem. I wonder if the memory spring needs replacing also. Seal is size BS012.
But a new seal did reduce the temp the boiler was sealing from 120Cish to 108C, so that drain port doesn’t steam for long. I can live with it, but will replace the valve at some stage.
When you refit, add 4 or 5 turns of PTFE to the thread and add a new zip tie to the drain hose on the bottom.
