. Probably not 1) Air will flow through spaces (eg, piston-to-cylinder-wall) that water (or hydraulic oil) won't. 2) Water provides some cooling for the piston, cylinder, and head. Air doesn't work as well (and air will heat up significantly when compressed). 3) Water provides some lubrication; air will not. 4) The output volume, using air, will be very low. Water is not compressible, so it's volume doesn't change when pumped. Even if you can get it to pump air reliably, you're not going to get much volume compared to water.