Master cylinder for electronic brake system

A master cylinder for an electronic brake system, comprising:

• a cylinder block having a first master chamber pressurized by a first master piston and a second master chamber pressurized by a second master piston, the first master piston moving relative to an operation of a brake pedal;
• a first hydraulic port formed in the cylinder block to provide a pressing medium of a reservoir to the first master chamber;
• and
• a pedal simulator provided between the first master piston and the second master piston and having a rubber member that provides reaction force to the brake pedal,
• wherein the first master piston comprises:
• a first piston and a second piston coupled to be movable relative to each other and having a communication channel to allow the pressing medium to flow between the first hydraulic port and the first master chamber;
• a communication channel sealing member installed between the first piston and the second piston to block a portion of the communication channel according to a relative movement of the first piston and the second piston;
• and
• an elastic member configured to provide elastic force in a direction in which the first piston and the second piston are spaced apart to prevent the communication channel from being blocked by the communication channel sealing member.