the force of the return spring 63 in this embodiment the fluid flows through the inlet port 61 into the pressure chamber 60 and flows outside through the outlet port 62 therefore water weeds or trash contained in the fluid is discharged instantly and hence the cylinder actuator 55 will not malfunction a pin 64 is attached to the free end of the piston rod 56 so as to project laterally and a lever 66 provided with a slot 65 is fixedly attached to an end portion of the shaft 47 the pin 64 of the piston rod 56 is inserted in the slot 65 of the lever 66 to turn the shaft 47 through the lever 66 when the piston rod 56 is moved if the pressure in the pressure chamber 60 increases beyond the predetermined level and the piston rod 56 is further projects from the cylinder 57 the lever 66 is turned clockwise as viewed in fig5 so that the exhaust control valve 40 opens the exhaust port 39 at an opening ratio of 100 if the pressure in the pressure chamber 60 decreases below the predetermined level and the piston rod 56 is retracted the lever 66 is turned counterclockwise.