A hollow piston pump (10) for pumping animal waste material including paddles (55, 155, 255) to agitate waste material collected in a hopper (11) and aid in its transfer into the hollow piston (17), a pumping chamber (14) including a cylindrical tube (76), and/or a cylindrical hollow piston (17) having...http://www.google.es/patents/US4445823?utm_source=gb-gplus-sharePatente US4445823 - Animal waste pumping system
