I have come across a similar system in a office block.
How we think we solved the problem was to descale the mechanism.
With the air pump activating the mechanism, the moving parts must move freely and this was acheived by cleaning the plastic parts with a descaler liquid.
Pour some done the centre and manualy slide the plunger up and down a few times.
It then should flush with one press of the button.
Cliff Crouch
October 2008