Hydraulic control unit and hydraulic motor vehicle braking system

A hydraulic control unit for a vehicle, including a hydraulic fluid reservoir with a hydraulic fluid reservoir space, out of which a hydraulic pressure generator is able to suction hydraulic fluid, an air space and a piston movingly accommodated in the hydraulic fluid reservoir, which separates the hydraulic fluid reservoir space and the air space from each other, wherein the air space is connectable and/or connected to an interior space of the hydraulic control unit, and a hydraulic motor vehicle braking system.