单项选择题
A.one valve seats upwards, while the other seats downwards
B.the reed valves used on the discharge are made substantially thicker and heavier than the suction valves
C.the suction valve springs exert a greater tension than the discharge valve springs
D.the discharge valve springs exert a greater tension than the suction valve springs
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（）
单项选择题 Although lube oils used in the main lubricating service systems should have a relatively high flash point to avoid ignition, they can create smoke and fire hazards when they（）