A brake mechanism comprises a cylinder body having a large chamber and a small chamber, a power piston movably positioned in the large chamber and dividing it into a control chamber and a vacuum chamber in communication with each other, a hydraulic piston and a control valve having a communication orifice both of which are positioned in said small chamber of the cylinder body and moved by a power piston to divide the small chamber into a high pressure chamber communicating with a driving wheel brake cylinder, a buffer chamber having a communication opening for selectively communicating the buffer chamber with a brake cylinder in accordance with the operation of the control valve and a control chamber, a relay valve cylinder communicating at one end with air and at the other end with the vacuum chamber and having a communication orifice open to the control chamber in the large chamber, a relay valve piston provided within a relay valve cylinder such that it can be operated by a movable core of an electromagnet for opening and closing the communication opening on the relay valve cylinder, a detecting means connected to the solenoid of the electromagnet through a servo amplifier to detect the locking of the driving wheels, and a communicating means for communicating the non-driving wheel brake cylinder with a master cylinder.