A reciprocating drive mechanism having a housing with upper and lower internal chambers with a spool slidably positioned inside the upper internal chamber. There is at least one fluid inlet and fluid exhaust communicating with the upper internal chamber and at least one slide valve positioned within the upper internal chamber and traveling with the spool. A piston positioned in the lower internal chamber divides the lower internal chamber into an upper and lower cylinder space. There is at least one fluid conduit communicating between the upper internal chamber and the upper cylinder space and at least one fluid conduit communicating between the upper internal chamber and the lower cylinder space. A valve stem is connected to the piston and includes a bore communicating with the upper internal chamber and an exhaust passage is positioned between the upper and lower internal chambers. A first side passage is formed on the valve stem and connects to the bore in the valve stem and a second side passage formed in the valve stem below the first passage and shaped to bridge a seal positioned between the exhaust passage and the upper internal chamber.