A master cylinder device which supplies a pressurized brake fluid to a brake device provided in a wheel and actuated by a pressure of the brake fluid, comprising:
- a housing whose front side is closed;
- a pressurizing piston having a blind hole opening at a rear end thereof, which includes a main body portion, and a flange portion formed around an outer circumference of the main body portion, and which is disposed in the housing such that:
- in front of the main body portion, there is defined a pressurizing chamber for pressurizing the brake fluid to be supplied to the brake device;
- in the rear of the flange portion, there is defined an input chamber into which a brake fluid is introduced from a high pressure source and which has a ring shape;
- and, in front of the flange portion, there is defined an opposing chamber filled with a brake fluid, opposing to the input chamber with the flange portion interposed therebetween, and having a ring shape;
- an input piston which is fitted in the blind hole of the pressurizing piston such that, in front of the input piston and between the input piston and the pressurizing piston, there is defined an inter-piston chamber filled with a brake fluid, which is connected to an operation member at a rear end portion thereof, and which is allowed to shrink by operating the operation member;
- a reaction force generating mechanism for generating an elastic reaction force against the shrink of the input piston;
- and
- an inter-chamber communication passage for making the opposing chamber and the inter-piston chamber communicate with each other so as to enable a mutual absorption of respective volume changes of the opposing chamber and the inter-piston chamber, the volume changes being caused by a forward and a rearward movements of the pressurizing piston,
- wherein the master cylinder device is configured such that, normally, in a state in which the opposing chamber and the inter-piston chamber communicating with each other by the inter-chamber communication passage are hermetically closed and an operation force applied to the operation member is not transmitted from the input piston to the pressurizing piston, the pressurizing piston pressurizes the brake fluid in the pressurizing chamber depending on a pressure of the brake fluid introduced from the high pressure source while the elastic reaction force generated by the reaction force generating mechanism is permitted to serve as an operation reaction force against an operation of the operation member,
- wherein the master cylinder device further comprises an operation-force-dependent-pressurization realizing mechanism which, under a condition in which the pressure of the brake fluid introduced from the high pressure source is insufficient, allows the transmission of the operation force from the input piston to the pressurizing piston, thereby realizing a pressurization of the brake fluid in the pressurizing chamber that depends on the operation force, and
- wherein the operation-force-dependent-pressurization realizing mechanism includes:
- a low-pressure-source communicating device for making the opposing chamber and the inter-piston chamber communicate with a low pressure source so as to allow the input piston to come into abutting contact with the pressurizing piston;
- and
- an input-piston-shrink prohibiting mechanism for prohibiting the shrink of the input piston.