The present invention concerns a hydraulic system comprising a hydraulic pump (130) installed on a truck tractor (10), a hydraulic pipe (160) connected to the outlet of the pump (130) and a connector (170) positioned at the outlet of the pipe (160) and designed to be connected to a complementary socket (180) provided on a trailer (20) in order to supply a ram (110) positioned on the trailer, for example for tipping purposes, characterised by the fact that it comprises a circuit selector (210) designed to connect said socket (180) selectively either to a ram (110) or a conduit (220) supplying a drive assistance circuit and an assistance circuit return conduit (250) designed to be connected to a tank (120) positioned on the truck tractor.