A hydraulic cylinder for a brake system includes a cylindrical body and a piston. The cylindrical body has a receiving space longitudinally defined therein for movably and sealingly receiving the piston. The receiving space is divided into a first chamber and a second chamber diametrically smaller than the first chamber. The piston has a first portion and a second portion. The first portion and the second portion sealingly and movably abut against the first chamber and the second chamber respectively. In a speed reduction stage the first portion interacts with the first chamber, and in an emergency brake stage the first portion and the second portion interact with the first chamber and the second chamber respectively for providing a greater braking force than in the speed reduction stage.