Vehicle height control system

A height control system for a vehicle having a plurality of wheels, comprising:

• an extendable and retractable hydraulic cylinder associated with each of said wheels for controlling the height of the vehicle by retraction of said cylinder;
• a source of pressurized oil, including a reservoir tank and a pump having an outlet port;
• a main line connected to said outlet port of said pump;
• each said hydraulic cylinder having a branch line extending therefrom and connected to said main line;
• a solenoid-operated charge-discharge valve in each said branch line having a first position blocking said branch line and a second position permitting oil to flow through said branch line in opposite directions;
• each said hydraulic cylinder having a plurality of gas springs connected in parallel therewith;
• a solenoid-operated oil discharging valve connected between said main line and said reservoir tank and having a first position disconnecting said main line from said reservoir tank and a second position communicating said main line with said reservoir;
• a pilot-controlled directional control valve for each said cylinder, having a pilot chamber, and arranged such that, when pressurized oil is charged in said pilot chamber, said control valve takes one of a first position in which said plurality of gas springs are communicated with said hydraulic cylinder and a second position in which at least one of said gas springs is at least substantially operably disconnected from said hydraulic cylinder, and when oil is discharged from said pilot chamber, said control valve takes the other of said first and second positions;
• each said pilot chamber having a pilot line having one end connected thereto and another end connected to one of said main line and said branch line at a position between said pump and said charge-discharge valve;
• and
• means for controlling said solenoid-operated valves so that oil is discharged from a said hydraulic cylinder by placing both of said solenoid-operated charge-discharge valve and said solenoid-operated oil discharging valve in said second positions, oil is supplied to a said hydraulic cylinder by placing said solenoid-operated oil discharging valve in said first position and placing said solenoid-operated charge-discharge valve in said second position while said pump is energized, oil is discharged from said pilot chamber by placing said solenoid-operated oil discharging valve in said second position, and pressurized oil is charged in said pilot chamber by placing said solenoid-operated oil discharging valve in said first position while said pump is energized.