The working principle of the hand brake valve is to pull the brake pedal when pulling the brake valve press the balance spring down. Through the balance spring and the lower seat and the steel ball press the balance arm through the push rod and the steel ball to push the two cavity diaphragm assembly down. After eliminating the clearance first close the exhaust valve mouth and then open the inlet valve port. The compressed air in the air storage cylinder enters into the brake air chamber through the brake valve. The push rod pushes the adjustment arm to rotate the cam and the brake shoe is pressed to the brake drum produce braking effect.