of valve 41 a second input to shuttle valve 50 is supplied by way of an air line 52 leading from a second shuttle valve 53 line 52 also leads to a pilot port of valve 42 one input of valve 53 is supplied by an emergency return valve 54 which has an input connected to an air supply inlet 37c the second input to valve 53 is supplied by an air line 55 leading from the output port of depth valve 38 a remote start valve 56 has an input port connected to an air supply inlet 37d and an output port connected by way of an air line 59 to a second pilot port of control valve 42 in the preferred embodiment described herein valve 56 is a mechanically activated and normally closed three way valve with spring return control unit 14 is comprised of a piston 14d and an internal spring 14e which normally urges the piston to the left the piston is bypassed by an adjustable flow restriction 14f operating in parallel with a check valve 14g travel of the piston in the feed direction thus requires fluid to pass through the restriction 14f thereby controlling the rate at which piston 30b can drive the motor 33 to urge the chuck.