The invention relates to a hydraulic hybrid drive system (1) for a motor vehicle, comprising driven wheels (2), which have friction brakes (3) for applying a friction braking torque to the driven wheels (2), and a continuously adjustable hydraulic transmission (4), which has a motor-side hydraulic machine (5) and a wheel-side hydraulic machine (6) having a direct fluid connection to the motor-side hydraulic machine, wherein the hydraulic machines (5, 6) are connected in series and to the driven wheels (2) of the motor vehicle in regard to driving, such that the hydraulic machines (5, 6) can be operated both as hydraulic pumps and as hydraulic motors, and at least one hydraulic fluid energy storage device (7), which is connected to the hydraulic transmission (4) in regard to fluid flow, wherein the motor-side hydraulic machine (5) is connected to the wheel-side hydraulic machine (6) in regard to fluid flow by means of a high-pressure hydraulic line (8) and a low-pressure hydraulic line (9). The invention further relates to a method for operating a hydraulic hybrid drive system (1).
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