A system is provided for controlling the balance piston pressure in a screw compressor. The system can use the slide valve of the compressor as a valve to control the flow of fluid from a fluid source to the balance piston. When the slide valve prevents direct flow between the fluid source and the balance piston, an alternate path is used to provide fluid at a reduced pressure to the balance piston. The reduced pressure fluid is obtained by passing the fluid from the fluid source through an orifice to lower the fluid pressure.