look at the drawing on the site and look inside the boiler where the big brass bit hes a large circle on the end there is a diaphram inside (dont even think about it ) it could be split or streched would cause you problems also there is a filter/flow restrictor in the cold water pipe this may be clogged with sediment try to trace it and clean it
bahaco i owe you an apology mate i was wrong slotted
bahaco i owe you an apology mate i was wrong slotted