A spray system comprising: a pre-pressurized fluid source (22, 24, 26) supplying pre-pressurized spray fluid; a proportioning cylinder having opposite first and second axial extents; a valved inlet line that fluidly connects the pre-pressurized source near the first axial extent of the proportioning cylinder in a first state of the spray system, and near second axial extent of the proportioning cylinder in a second state of the spray system; a sprayer (28, 30) fluidly connected to the proportioning cylinder via a valved outlet line; and a double-action reciprocating plunger situated within the proportioning cylinder with a first axial surface facing the first axial extent of the proportioning cylinder, and a second axial surface facing the second axial extent of the proportioning cylinder, characterized in that: the first and second axial surface have equal working surface areas; the double-action reciprocating plunger is anchored to a displacement rod that extends the full axial length of the proportioning cylinder regardless of the position of the double-action reciprocating plunger within the proportioning cylinder, and the displacement rod comprises a first section extending from the first axial surface of the double-action reciprocating plunger through a first bearing seal at the first axial extent of the proportioning cylinder to engage a motor, and a second section extending from the second axial surface of the double-action reciprocating plunger through a second bearing seal at the second axial extent of the proportioning cylinder and into a rod reservoir. a proportioning cylinder having opposite first and second axial extents; a valved inlet line that fluidly connects the pre-pressurized source near the first axial extent of the proportioning cylinder in a first state of the spray system, and near second axial extent of the proportioning cylinder in a second state of the spray system; a sprayer (28, 30) fluidly connected to the proportioning cylinder via a valved outlet line; and a double-action reciprocating plunger situated within the proportioning cylinder with a first axial surface facing the first axial extent of the proportioning cylinder, and a second axial surface facing the second axial extent of the proportioning cylinder, characterized in that: the first and second axial surface have equal working surface areas; the double-action reciprocating plunger is anchored to a displacement rod that extends the full axial length of the proportioning cylinder regardless of the position of the double-action reciprocating plunger within the proportioning cylinder, and the displacement rod comprises a first section extending from the first axial surface of the double-action reciprocating plunger through a first bearing seal at the first axial extent of the proportioning cylinder to engage a motor, and a second section extending from the second axial surface of the double-action reciprocating plunger through a second bearing seal at the second axial extent of the proportioning cylinder and into a rod reservoir. the first and second axial surface have equal working surface areas; the double-action reciprocating plunger is anchored to a displacement rod that extends the full axial length of the proportioning cylinder regardless of the position of the double-action reciprocating plunger within the proportioning cylinder, and the displacement rod comprises a first section extending from the first axial surface of the double-action reciprocating plunger through a first bearing seal at the first axial extent of the proportioning cylinder to engage a motor, and a second section extending from the second axial surface of the double-action reciprocating plunger through a second bearing seal at the second axial extent of the proportioning cylinder and into a rod reservoir.