A completely assembled air shut-off control valve unit illustrated as fitted in a bore of a pneumatically powered nut runner in the path of air flow between the throttle valve and the motor. The unit is of capsule form defined by means of a shell in which the various components are confined. A cap closes over an open end of the shell through which end the various components may be withdrawn. The valve is of a piston and rod form having a spring load in association with an adjusting screw and travel nut means for adjusting the load of the spring upon the valve. Structural design of the valve, together with a valve guide serve to effect pneumatic balancing of the valve and consequent reduced friction in its movement.