Abstract: A power booster assembly for use to increase the power used in pulling or driving tool mechanism such as used in straightening an automobile frame and the like. A cylindrical body having closed ends and attachment means at each of the ends with a power piston mounted for reciprocation therein together with a spring biased hammerhead which is latchable on to one end of the power piston for fluid actuation of said piston to draw the hammerhead against the spring bias. Cam release structure within the body causes the hammer latch mechanism to be released and permits the hammerhead to trip and bang against one closed end of the cylinder with tremendous force. Repeated action of catching and releasing the hammerhead takes place according to the amount of fluid being permitted into the power cylinder. Valve mechanism is arranged to automatically cause proper reciprocation of the power piston according to the amount of fluid and the pressure thereof being fed to the assembly.