Apparatus for producing a constant flow, constant pressure chemical vapor deposition includes a manifold having inlet valves for simultaneously switching equal flows of reactive and nonreactive gas between a process chamber and a vent chamber. A constant flow through the process chamber during a deposition is maintained by replacing each reactive gas flow with the equal nonreactive gas flow. Substantially equal pressures within each chamber are maintained by the substantially equal flows of gases. Any "dead space" within the manifold downstream of the inlet valves is minimized by use of a radial manifold. The nonreactive gas flows also purge the "dead space" of reactive gas that may linger after its flow is switched to the vent chamber.