I guess the black bellows inside the air button assembly, part number 38488 000, is gone. I found a 'Bellows service kit', i.e. black bellow,which is available on spares websites.
How does one get the black rubber bellows out and in? It is tightly contained within the white threaded pipe at the back of the push button.
I also cannot remove the clear air tube from the black bellow. Very tight.