Abstract: The invention relates to a rock drill air leg control system incorporated in the backhead (10) of a rock drill. The system comprises a single spool valve (22) having two manual controls (46, 53), one for setting a trust applied by an air leg to a rock drill and the other for retracting the air leg without affecting the thrust setting achieved by operation of the first control. The first manual control is in the form of a twist-grip control (53) which is arranged to set the axial position of a spring-biased valve spool (30) in the valve (22) via a pin (50) and a plunger (46). The plunger (46) also forms the second of the manual controls and can be pressed against the spring-biased valve spool (30) to vary the axial position of this spool.