may alternatively fully close the cavity openings 42 and 44 in particular in the position of fig1 the peripheral face 54 corresponds to the angular extent of the wall of valve chamber 40 extending between the openings 42 48 however if the valve element 52 of fig1 were rotated 45 u00b0 clockwise to extend substantially vertical with respect to fig1 the peripheral face of the valve element 52 would fully close the openings 42 and 44 preferably with the seals 56 being exterior to the periphery of the openings 42 and 44 to improve the closure of these openings the exterior faces of valve element 52 which extend between the peripheral face 54 are formed with a concave configuration such that the central portion of the valve element has a reduced thickness compared to the peripheral face 54 the reason for this concave configuration will be made clear below a valve motor 58 is fixed to the outer casing 12 and operatively connected to the valve pivot rod 50 for rotation of such rod in the embodiment shown the motor 58 is formed as a hydraulic motor and includes appropriate hydraulic lines 60 for supplying fluid to the motor the lines 60 are in communication with a valve switching valve 62 which will alternately connect one or the other of the lines 60 with a main hydraulic fluid supply line 64 the switching valve 62 may be controlled by hydraulic fluid and in particular the switching valve 62 is controlled by the rotation sensor 30 of motor 22 as such the switching valve 62 may be operatively connected to the.