Grohe push button flush?

I have a Grohe concealed cistern, it stopped flushing and so I replaced the bellow in the button flush. But it doesn't give enough air pressure to make it flush. The only way to make it slush is to use a 50 ml syringe (at full draw back) on the tube. It's about 19 years old now and I would have to rip out a wooden wall to get to the cistern (someone's daft idea not to put in access). Any ideas anyone? Thanks.