Variable Displacement Pump

Here's a bunch of links on swash-plate piston pumps:

http://www.google.com/search?sourceid=ie7&q=swash+plate+pump&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&rlz=1I7ACEW_enUS408US408

Alternatively, here are some variable volume vane pumps:

http://www.google.com/search?sourceid=ie7&q=variable+volume+vane+pump&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&rlz=1I7ACEW_enUS408US408

You could also search http://www.denisonhydraulics.com/ I worked on a 12 x 100HP unit of theirs. It was built like a tank. The 12 pumps were all swash-plate piston pumps. Never a problem until they were over 25 years old. We replaced two of the pumps with Rexroth 100HP units, they proved to be very reliable. Denison were terrible with repairs.