to be reshifted to its supply position the next morning control valve 14 is shown as being a four way valve in order to control the two lines 19 and 21 leading to the left and right hand sides respectively of motor 15 in its illustrated position control valve 14 supplies pressure unrestricted through a one way check valve 22 to the right hand side 23 of motor 15 shifting part 16 to the left at the same time air will leave the left hand side 24 of motor 15 through a restriction 25 to exhaust when valve 14 is shifted the opposite conditions will prevail air will flow unrestricted through check valve 26 to chamber 24 of motor 15 and will leave chamber 23 through restriction 27 to exhaust control valve 14 will conventionally rest in one or the other of its positions such as the position shown in fig1 being shifted to its other position by means 28 such as a solenoid or pilot valve without the presence of safety valve 11 when supply line 12 is shut down by moving valve 13 to its exhaust position all pressurized air will leave the system including chambers 23 and.