A hydraulic brake system includes a piston that separates a first pressure chamber from a second pressure chamber of a cylinder bore. A valve within a piston bore is movable between three positions. In the first position, the first and second pressure chambers are in fluid communication with a low-pressure fluid supply; in the second position, the second pressure chamber is isolated from the first pressure chamber and the low-pressure fluid supply; in the third position, a high-pressure fluid supply is in fluid communication with the second pressure chamber, allowing high-pressure fluid to flow into the second pressure chamber. High pressure in the second pressure chamber moves the boost piston, applying boosted braking pressure to a vehicle brake. Pressure in the second chamber against a surface of the valve biases the valve toward the first position. An accumulator stores pressurized fluid and provides the high-pressure fluid supply, enabling on-demand brake boosting force during engine-off operation of a hybrid vehicle.