Hydraulic platform lift incorporating positive displacement valve, and positive displacement valve for hydraulic platform lift

A hydraulic platform lift for use with a truck or truck trailer, comprising:

• a platform member having two opposite side edges;
• first and second hydraulic cylinders each having a piston member therein, each of said first and second hydraulic cylinders operatively coupled to a respective side edge of said platform member to permit raising and lowering of said platform member;
• pump means for supplying pressurized hydraulic fluid to said hydraulic cylinders via a positive displacement means;
• said positive displacement means comprising a pair of juxtaposed cylinder members, each containing a longitudinally-moveable piston, such pistons operatively coupled to each other so that movement of one piston causes an equal movement of the other, each cylinder member having a pair of opposite ends, one end of which having an aperture to permit ingress of hydraulic fluid from said pump means, and another aperture at an opposite end to permit egress of hydraulic fluid from said cylinder member to one of said hydraulic cylinders;
• and
• said positive displacement means adapted, when said pump means provides pressurized hydraulic fluid via a respective aperture to a first side of each of said pistons, to cause said pistons to be displaced an equal distance causing an equal egress of hydraulic fluid from said cylinder members via a respective aperture from a side of each of said pistons opposite said first side and a corresponding equal supply of hydraulic fluid to each of said first and second hydraulic cylinders with resultant equal movement of said piston members therein regardless of different loads which may be applied to said first and second hydraulic cylinders.