Abstract: A pneumatic system is used for controlling the position of a hydraulically-actuated drilling choke to limit the maximum pressure in the casing of an oil well. Pressure in the casing is used directly to switch a converter valve that applies air pressure to the pressure limiting system. The air pressure opens the choke to relieve casing pressure. The system traps a volume of air at a pressure corresponding to the choke position and uses this pressure as a memory signal for resetting the choke to its preset position after casing pressure has decreased below its allowable maximum. The system also uses pneumatic pressure to assure that the memory is not lost before the choke is reset.