A hydrostatic drive system for a vehicle (10) comprises an open circuit (100), a closed travel circuit (120), an electronic controller (320), and a main variable displacement hydraulic pump (102) driven by an internal combustion engine (12) equipped with a speed sensor (350). The main variable displacement hydraulic pump may be configured to supply hydraulic fluid to a hydraulic consumer (34, 44, 50) via a hydraulic supply line (106). A pressure relief valve (118) may be connected to a reservoir (300) and may be permanently connected to the hydraulic supply line of the open circuit. The electronic controller may be configured to run a strong braking mode in which the electronic controller at least partially increases the displacement of the main variable displacement hydraulic pump such that hydraulic fluid is discharged via the permanently connected pressure relief valve into the reservoir if the sensed engine speed is higher than a predetermined maximum engine speed.