Weak flush - pneumatic button mech or flush valve?

[Brix]

I have a wall hung pan with a concealed cistern. The flush is the "old style" Grohe Eau2 dual flush pneumatic button. When pressed the flush valve lifts lightly and closes giving a very short weak flush. You can manually lift the flush valve in the cistern and it moves freely, flushes fine and closes normally. Connecting air hoses are intact. Is the fault likely to be in the button mech or a problem with the valve.

Also can anyone recommend a single a single flush button that is a direct replacement for the dual flush Grohe Eau2 "old style" - ideally same diameter. The original is rubbish and the buttons easily snap off. Grohe say there is none (not even the new version of the Eau2).

 

[Basher]

You might check whether the flush button is the source of the problem by disconnecting the hoses at the valve end, blocking the airflow (eg finger over the ends of the hoses) and operating the button. It should resist your 'push', probably in a slightly 'spongey' way! If you can push the button right in, there's a leak!
Spares for the old style Eau2 do seem to be available.
Don't know of an equivalent I'm afraid.