A sash balance brake assembly for a sash window system is disclosed. The sash window system includes a window frame having a pair of generally parallel tracks. Each of the tracks has an outer wall, front and back intermediate walls and front and back inner walls collectively defining a channel. A sash window is transversely disposed between the tracks and is adapted for slidable movement along the tracks. A pair of sash balance assemblies provide a generally upward bias on transverse sides of the sash window. The sash balance brake assembly comprises a rotor rotatable about a rotor axis, a bolt having a generally planar engaging surface, a slider body for placement in a respective one of the channels and for coupling to a respective one of the sash balance assemblies. The slider body includes an aperture for rotatably supporting the rotor adjacent the bolt. The rotor and the bolt include cooperative camming surfaces for converting rotary motion of the rotor into movement of the bolt along the rotor axis, such that the bolt engaging surface selectively engages and disengages the track outer wall.