Hydraulic control unit and hydraulic motor vehicle braking system

A hydraulic control unit for a vehicle, comprising:

• 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 piston defines a first piston surface and a second piston surface, wherein the air space is connectable and/or connected to an interior space of the hydraulic control unit;
• and wherein the second piston surface is defined by a circumferential flange extending downwardly from a peripheral section of the piston, wherein the circumferential flange serves to block the air space from connecting to the interior space of the hydraulic control unit when the hydraulic control unit is in a charged condition, while air is trapped between the second piston surface and an end cap, wherein a base surface of the end cap extends beyond a surface of a body of the hydraulic control unit, wherein the hydraulic fluid reservoir is a low-pressure reservoir that is not acted upon by any spring force, and wherein the air space and the interior space are hermetically sealed from a surrounding atmosphere by a sealing element such that the hydraulic control unit remains operational when submerged.