Abstract: On his rest day at the job site, Employee #1, an apprentice operating engineer, was assigned to move a Rex hydrostatic vibratory roller. This involved going down a gravel road with a slope of up to 12 percent, making a 90 degree right turn along the steep section, and going down another gravel road to the worksite. Employee #1 was moving downhill, apparently running in low gear on engine compression braking, when the pressure in the hydraulic drive system became excessive due to the slope and/or the positions of the throttle and control valve. The system bypass valve opened, causing the roller to lunge forward or buck. In an attempt to control the lunging action, Employee #1 started moving the direction level, applying the brakes, and throttling down. This apparently stalled the engine, deactivating the power-assisted steering. Employee #1 lost control of the roller and it overturned uphill on a side slope. He sustained serious injuries in jumping clear of the vehicle.