Bad Hadraulic Lifters, Maybe?

yeah,, it's a possitive dissplacement pump.... even run dry,, they suck.. something jambed it..

Internal Gear Pump Overview
The internal gear pump is non-pulsing, self-priming, and can run dry for short periods. They're also bi-rotational, meaning that the same pump can be used to load and unload vessels. Because internal gear pumps have only two moving parts, they are reliable, simple to operate, and easy to maintain.

http://www.pumpschool.com/principles/internal.asp