Worcester Bosch 28CDI pressure drop. Repair?

either the pressure relief valve or "dump valve" as you called it is passing or the expansion vessel on the back of the boiler is flat and needs pumping up./ open the pressure relief valve, and use a foot pump the pump up the expansion vessel on the back of the boiler. its the big red disc on the backl opf the boiler.

craig plymouth
May 2006