A spray nozzle assembly comprising: a nozzle support body having a liquid supply port for coupling to a liquid supply, said nozzle support body further having an air supply port for coupling to a pressurized air supply and air outlet port; a spray nozzle supported by said nozzle support body; a piston having said piston head supported for movement in an opening of said nozzle support body with a downstream end of the piston being selectively engageable with a spray nozzle liquid supply inlet for controlling the flow of liquid to and discharge from said spray nozzle; a return spring supported within a spring chamber in said nozzle support body opening on an upstream side of said piston head for biasing said piston to a spray nozzle liquid supply inlet closing position; said nozzle body opening defining a piston chamber on a downstream side of said piston head opposite of said spring chamber; said nozzle support body having an air passage system communicating with a control valve characterized by; said control valve being selectively operable for enabling communication of pressurized air from said nozzle support body air supply port to said piston chamber for moving said piston to a spray nozzle liquid supply inlet open position against the biasing force of said return spring for enabling the flow of liquid to and discharge from said spray nozzle and for preventing the flow of pressurized air through said air passage system from said nozzle support body air supply port to said piston chamber while enabling the communication of pressurized air from said piston chamber to and through said spring chamber to said nozzle support body air outlet port causing an elevated pressure within the spring chamber acting on the piston head resulting in a momentary force supplementing the biasing force of said return spring in moving said piston toward said spray nozzle liquid supply inlet closing position. a nozzle support body having a liquid supply port for coupling to a liquid supply, said nozzle support body further having an air supply port for coupling to a pressurized air supply and air outlet port; a spray nozzle supported by said nozzle support body; a piston having said piston head supported for movement in an opening of said nozzle support body with a downstream end of the piston being selectively engageable with a spray nozzle liquid supply inlet for controlling the flow of liquid to and discharge from said spray nozzle; a return spring supported within a spring chamber in said nozzle support body opening on an upstream side of said piston head for biasing said piston to a spray nozzle liquid supply inlet closing position; said nozzle body opening defining a piston chamber on a downstream side of said piston head opposite of said spring chamber; said nozzle support body having an air passage system communicating with a control valve characterized by; said control valve being selectively operable for enabling communication of pressurized air from said nozzle support body air supply port to said piston chamber for moving said piston to a spray nozzle liquid supply inlet open position against the biasing force of said return spring for enabling the flow of liquid to and discharge from said spray nozzle and for preventing the flow of pressurized air through said air passage system from said nozzle support body air supply port to said piston chamber while enabling the communication of pressurized air from said piston chamber to and through said spring chamber to said nozzle support body air outlet port causing an elevated pressure within the spring chamber acting on the piston head resulting in a momentary force supplementing the biasing force of said return spring in moving said piston toward said spray nozzle liquid supply inlet closing position.