One difficulty in designing a pump/motor has to do with valve design. To eliminate pressure trapping yet seal and separate the ports is difficult. Keeping the valve plate from leaking or having too much contact pressure is another problem.
The EPA has done pump motor development for their hybrid drive so try contacting them.
I’ve been working on an electro-hydraulic pump motor to solve many of the problems inherent to current axial piston pump/motors but the project is currently stalled due to a lack of funding and time. Check the DEHydS link for more information.
