to the piston cylinder motors motor 52 for example becomes sufficient to stop motion thereof waste of energy and undue heating of the hydraulic fluid is thus substantially reduced while shown only in an illustrative and schematic manner valves 64 66 114 and 116 contain conventional relief valves other relief valves are indicated at 136 the pressure at which the unloading valves 106 and 108 are triggered is below the pressure relief point of these relief valves thus the relief valves never operate at the combined flow rate of both pumps during normal digging operation when an overload causes stalling it is only the affected first pump 70 or 76 which will be delivering full volume through the relief valves and the energy so wasted will be only the energy lost form that pump while first and second pumps 70 and 72 have been shown driven by a common shaft 74 from a prime mover this arrangement is not essential to the invention four individually driven pumps or any combination of multiple pumps would serve equally well pumps 70 and 76 could be of either fixed or variable displacement but the circuitry of fig3 was described above as if they were fixed displacement pumps the modified form of the invention as shown and illustrated in fig4 and.