[Kevin Sevcik]

It is legal. The minimum system is a set of gauges to show stored and working air pressure, and a relief valve to vent any stored pressure to atmosphere. You would also have to put the 60psi main regulator between the compressor and the pressure switch to turn off the compressor at 115 psi.

As eshteyn said, you'd really want a volume tank in there, though, or you'd have to wait for the compressor to fill up the piston each time you moved it. If you're worried about weight and you're only using a few small pistons, you might consider only using 4 volume tanks and keeping the compressor off your robot. You'd have a limited amount of air and number of piston firings to work with, but if you know how much you need precisely enough, you could make it work.