Demo and assembly guide: https://youtu.be/5G4Wwa94_aM
Exploded view in 3d viewer: https://skfb.ly/6QL8R

Extra parts needed:

• Nitrile glove
• Air balloon

Takes 4m of filament to print (about 5h at 20mm/s on my snail of a printer).
Will print a bit prettier with supports (80deg. overhang, touching buildplate) and ironing.
Parts with names starting with "X_" are optional.

• You can insert one balloon into another to get thicker walls. Meant for 2-walled balloon. Idles well with one (with a flywheel), but will hardly pull any vehicle.
• You can use metal M5 washer instead of printed one
• Found that PLA isn't the best material for the design: isn't very slippery, and "fluid" (the clips stop pushing the parts together with time)

Troubleshooting:

• Try using 2-walled balloon (one inserted into another)
• Hissing sound - air leaks heavily from balloon connection. Roll up the balloon lip to make it thicker.
• Can be turned freely by hand, but struggling when working - the inner valve may have too much friction against the frame (sanding the valve and the frame and some lubrication, maybe graphite, may help for a while).
• Jams on a certain cylinder - this glove finger may need a bit more slack.
• Can't be turned freely by hand - washer sits too tight on the crankshaft, or the hole for the LEGO shaft printed a bit too small.