An earth drilling apparatus has a pneumatic motor operated earth drilling tool, a valve assembly connected to the motor, and a drill string connected to the valve and to a source of pneumatic fluid. The drilling tool has an earth drilling bit and a rotor/stator motor section to apply a rotational force to rotate the bit at the bottom of the string. The valve assembly is disposed between the drilling tool motor and the source of pneumatic fluid and controls the flow of pneumatic fluid to the tool. The valve assembly has a pneumatic pressure-operated valve which is operable in response to the pressure of pneumatic fluid in the drill string to be opened to permit flow of pneumatic fluid to the tool at a predetermined operating pressure to transmit an initial pulse of pneumatic fluid to initiate operation of the motor. The valve is kept open at a lower pressure than that required to open it. The valve is a pressure operated valve, spring-loaded toward closed position, which opens at a first predetermined pneumatic pressure permitting flow to said tool and closes at a second, substantially lower, pneumatic pressure.