Patent ID: 7850248

Claim:
A hydraulic system useful in combination with a hoist for raising or lowering a truck bed to and from a truck frame, including the option of safely and controllably lowering said truck bed to said truck frame by gravity alone, said hydraulic system utilizing hydraulic fluid during operation, said hydraulic system comprising: an extendible and retractable cylinder for raising or lowering said truck bed, said cylinder comprising a cylinder shaft having a first end and a second end, said first end comprising a piston which moves inside a cylinder housing as said cylinder is extended and retracted; said piston comprising a valve means for regulating the flow of said hydraulic fluid utilized by said hydraulic system through said piston; said cylinder housing comprising a base end and a head end, said base end comprising a base port that allows said hydraulic fluid to flow into and out of said base end of said cylinder housing, said head end comprising a head port that allows said hydraulic fluid to flow into and out of said head end of said cylinder housing; said base port comprising a valve means so configured to regulate the flow of said hydraulic fluid through said base port; a reservoir for storing said hydraulic fluid, said reservoir having an inside and an outside, said inside containing during operation of said system a combination of said hydraulic fluid and air which renders said reservoir capable of being pressurized while lowering said truck bed, said reservoir comprising a first port and a second port through which said hydraulic fluid flows during said operation, said second port comprising a valve means that permits said hydraulic fluid to leave, but not enter, said reservoir, said reservoir further comprising a third port through which said hydraulic fluid can be added to said reservoir prior to said operation, said third port including during said operation a means for sealing said third port from said air leaving said reservoir and for sealing said third port from outside air entering said reservoir; a system control valve means fluidly connected to said reservoir and both said head end and said base end of said cylinder housing for regulating the flow of said hydraulic fluid between said reservoir and said cylinder housing; a pump for pumping said hydraulic fluid through said system, said pump fluidly connected to said system control valve means and said reservoir; said reservoir also fluidly connected directly to said head end of said cylinder housing; wherein said reservoir is so designed such that when pressurized while lowering said truck bed, hydraulic fluid flows from said reservoir and into said head end of said cylinder housing, whereby the combination of said pressurized reservoir and said hydraulic fluid present in said head end of said cylinder housing permits the option of said truck bed being safely and controllably lowered by gravity alone to said truck frame.