A malfunctioning accumulator or expansion tank that's lost it's air cushion.

[up2you2]

Does anybody know of who might replace an air cushion please?

 

The pump in question - see attachments.

[Crossy]

Let's keep all the responses in your other thread.

 

It may not be the accumulator, but looking at a couple of Googled images there appears to be an air valve on the accumulator to allow it to be pumped up.