crazy airing cupboard and (airlock?) shower problems

[MM2021]

Hi there,
I'd be very grateful for a second opinion on the following issue...

(1) My shower is *very* unreliable and seemingly getting worse. The cold water flow is good and reliable, but when I turn the mixer valve to hot, the flow will often reduce to a trickle. The "trick" to getting it working is to slowly turn the valve from cold to hot.

(2) There is very strong evidence that the shower is pulling in air and I think this must be the root of the problem. As I turn the shower from cold to hot, the valve will vibrate a bit and create a knocking sound, as though air is being forced through.

(3) Sadly, there seems to be an unlimited supply of air getting in. I'm quite sure there are no water leaks (based on my experiences with this house, I believe I would be able to hear a running water leak).

(4) the pressure gauge next to the filling loop in my airing cupboard consistently shows 1 bar.
I wonder if anyone has any thoughts as to what is going wrong here and what the solution might be? I bought this old house (90 yrs old) quite recently and am still learning about its issues...

Many thanks indeed for any help,

 

[oldbuffer]

1. Looks correctly fitted, and has the special flange on the cylinder through which the pump is fed.
2.I'd be tempted to check the shower first. As follows:
2a. Isolate hot and cold to shower.
2b. Remove shower bar by undoing the two nuts closest to the wall.
2c. Check any non-return valves / filters in the shower / wall connectors.
2d. Provided you can channel the water to avoid a flood, make sure the pump is turned on, then briefly turn the hot water back on. If you get a good jet out of the pipe, the pump is probably OK.
2e. Re-assemble and test again.