Pneumatic/hydraulic flush system(?) doesn’t apply enough force (air in hose just compresses)

Hi folks,

Moved into a place with this new toilet. It has a flushing system I haven’t encountered before. It uses a pneumatic/hydraulic button to push on a piston which raises an arm, opening the flush ‘gate’. Problem is (as I understand it) the hose is filled with air. When you push on the button the air just compresses, not outputting enough force to lift the gate fully open to flush.

Pictures:
https://imgur.com/a/6ScRgGZ

Pic 1: front of the panel w/ button, which sits on the wall covering the flushing system. (Brand of the button panel and toilet is VitrA)

Pic 2: reverse side of the panel. Note the wall behind, where the flush system is enclosed inside the wall.

Pic 3: Inside the wall. Red arrow points to the piston which pushes up on a little arm (green arrow) which in turn lifts open the vale/gate to flush the toilet.

Can I just fill the blue hose with water? I was going to use an irrigation syringe.

Thanks!