baxi 80 he combi boiler hot water fualt?

hi. my boiler hot water only works partially. the central heating is fine. i have partially traced the problem. the top diaphram pressure switch is not operating, it has 2 small copper pipes going to either side of the diaphram. if i operate the micro switch manaually it a starts working ok
the green neon on the tap symbol flashes but stays on if i hold micro switch in.
i opened the pipe on the side of the diaphram the stops it pushing out and there is a lot of pressure on it.

these pipes seam to be some sort of pressure balance system

cheers