单项选择题 The major difference between the discharge and suction valves installed in most low pressure, reciprocating air compressors is that（）

单项选择题 Which of the following problems will occur if the internal drain at either end of a hydraulic, two-way, spool-type directional control valve cylinder were to become plugged?（）

单项选择题 Hydraulically, servo-operated, automatic, change over valves, utilized in a two ram hydraulic steering gear, serve to（）