Hydraulic brake system

1. A hydraulic brake system for a vehicle, comprising:

a cylinder; and

a piston movably arranged inside the cylinder,

wherein the piston has a closed end, an open end and a piston wall surrounding a cavity of the piston between the closed end and the open end, the piston being movable along a longitudinal axis of the cylinder by hydraulic pressure within the cavity for pressing at least one brake pad against a brake disc,

wherein an outer diameter of the piston is smaller in a central section between the open end and the closed end than at the open end and at the closed end so that, in the central section, a space remains between the piston wall and an inner wall of the cylinder, and

wherein the hydraulic brake system further comprises a parking brake mechanism, wherein the parking brake mechanism comprises a spindle and a nut, the spindle being arranged along the longitudinal axis and protruding through the open end of the piston into the cavity and wherein the nut is engaged with the spindle to be movable along the longitudinal axis by a rotation of the spindle for applying or releasing a pressure on the piston and thereby activating or deactivating an integrated parking brake.

2. The hydraulic brake system of claim 1 , wherein the cavity is narrower in the central section of the piston than in at least one of a section between the central section and the open end and a section between the central section and the closed end.

3. The hydraulic brake system of claim 1 , further comprising a hydraulic fluid for exerting the hydraulic pressure within the cavity of the piston, wherein the space between the inner wall of the cylinder and the piston wall in the central section of the piston is free from the hydraulic fluid.

4. The hydraulic brake system of claim 1 , wherein a seal ring surrounding the piston closes a gap between the inner wall of the cylinder and the piston wall between the central section and the open end.

5. The hydraulic brake system of claim 1 , wherein a smallest value of the outer diameter of the piston in the central section is smaller than 0.8 times the outer diameter of the piston at the open end and at the closed end.

6. The hydraulic brake system of claim 1 , wherein a smallest value of an inner diameter of the cavity in the central section is smaller than 0.8 times a largest value of the inner diameter of the cavity at the open end and/or at the closed end.

7. The hydraulic brake system of claim 1 , wherein, in a plane cutting the nut perpendicular to the longitudinal axis, the cavity has a non-circular cross section.

8. The hydraulic brake system of claim 1 , wherein the nut fits through the central section of the piston to push against an end wall of the piston at its closed end for activating the parking brake.

9. The hydraulic brake system of claim 1 , wherein the nut does not fit through the central section of the piston and is arranged for pressing against a shoulder of the piston between the central section and the open end for activating the parking brake.

10. The hydraulic brake system of claim 1 , wherein the piston is formed in one piece and/or that at least a part of it is a cast part.

11. The hydraulic brake system of claim 7 , wherein the nut fits through the central section of the piston to push against an end wall of the piston at its closed end for activating the parking brake.

12. The hydraulic brake system of claim 7 , wherein the nut does not fit through the central section of the piston and is arranged for pressing against a shoulder of the piston between the central section and the open end for activating the parking brake.