A gas-actuated reciprocal drive apparatus has a double-acting piston in a pneumatic cylinder having a chamber at each end. Gas from an area of higher pressure in a compressed gas system flows into a first chamber, while the second chamber is in fluid communication with an area of lower pressure in the gas system. The piston moves toward the second chamber, purging gas therein back to the lower-pressure area in the gas system, without any venting to the atmosphere. A four-way gas valve reverses the piston motion after each stroke, by reversing the chambers' gas connections. The piston has a pair of circumferential seals, plus a differential shuttle valve that allows gas from the lower-pressure chamber to enter the annular space between the seals, such that the pressure differential across the seals always equals the pressure differential between the two chambers, regardless of the actual pressures in the chambers, thus reducing friction forces on the piston seals, increasing the power output of the apparatus, and extending the service life of the seals.