forward inertia of the piston carry it downwardly from position 3 illustrated in fig1 as valve member 70 moves upwardly to just above its fig1 position and where it strikes tappet 22 forcing the tappet to impact against the tool steel 23 driving the steel into the work as the pressure in the forward chamber 61 continues to rise with valve member 70 in its upper position illustrated in fig4 the piston 20 will be arrested as a bottom or forward dead center position illustrated approximately in position 4 in fig1 and the piston will immediately reverse itself and begin upward travel exhausting air in the rear or upper cylinder chamber through exhaust port 98 until it reaches a position not shown in fig1 where it closes exhaust port 98 and begins compressing air in the rear cylinder chamber 60 the piston continues traveling rearwardly upwardly from this position to a position near position 5 illustrated in fig1 where the rear end of the piston uncovers exhaust port 98 permitting the forward chamber 61 to be exhausted this causes a rapid decrease in pressure on the lower side of valve member 70 and with the increase in pressure on the top side of valve member 70 caused by compression in rear chamber 60 valve member 70 drops to its lower position sealed in the lower valve block member 68 illustrated in fig1 this causes inlet fluid to be directed again over the top of valve member 70.