Downhole shutoff tool

A downhole drilling valve tool, positioned above the drill bit, which utilizes a ball valve having a through bore for normal flow. The ball valve is operated bidirectionally by a piston driven mechanism between an open position and a closed position. The piston mechanism is responsive to a reference accumulator pressure on a first side and the pressure in the apparatus bore above the ball valve on a second side. Whenever the valve apparatus is lowered at least a minimum distance into the well bore, a pressure relief valve is used to maintain a fixed margin between the flowing bore pressure and the accumulator pressure. Closure of the ball valve occurs when the accumulator pressure exceeds the bore pressure during pump stoppages. The downhole drilling valve tool offers a more wear resistant sealing means, as well as higher pressure operability than conventional downhole check valves.