Hydraulic machine and reversible metering pump equipped with such a machine

The invention concerns a hydraulic machine comprising: a casing (14); a differential piston (1) having an upper ring (23) and a lower base (27), suitable for sliding in reciprocating movement respectively in the casing and in a cylindrical housing (22) inside the envelope, the piston separating two chambers (16, 17) of the casing; hydraulic switching means for supplying liquid to and discharging liquid from the chambers separated by the piston, said switching means being controlled by the movements of the piston and comprising at least one rod (4) acting on a distribution member (12), said distribution member comprising an upper valve (3) cooperating with a first seat (25) provided in the upper ring (23) of the piston and at least one lower valve (9) cooperating with a second seat (26) provided in the lower base (27) of the piston; and triggering means capable of causing, at the end of the stroke of the piston, a sudden change in the position of the switching means, under the action of an elastic means (E), to reverse the stroke. The lower and upper valve seats each respectively accept an upper frustoconical portion (25a; 26a) and a lower frustoconical portion (25c; 26c) tapering in opposite directions to each other. The invention also concerns a reversible metering pump comprising such a hydraulic machine. The invention comprises sealing means that are reversible with each other in order to enable to reversible function.