We have a conventional DHW system with a hot water tank running most
of the house. However the last owners added a pump to the airing
cupboard that supplies the en-suite basin and shower (this is above
the water tank so pressure is very poor otherwise.)

This is connected to the hot water tank using "Hep" style plastic
pipe that runs up into the attic and back down to the shower room. In
the attic there is a t-junction in each pipe and a short section
sticks up with a brass cylindrical object about 7cm tall and 3cm
across. On the top of these is a little plastic 'pen cap' that can be
screwed down or opened.

I've always assumed these are some soft of pressure release valve.
They've always had a bit of scale on them as if a very small amount of
water is released at times.

Last night one of them let out enough water to leave a wet patch on
the ceiling and blow all the paintwork on a section of picture rail.

Why do we need these valves? How should they work? How can i stop
them from soaking my ceiling?