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The brake master cylinder for a heavy weight vehicle which can offer the optimal braking effect and brake feel has been developed. This brake master cylinder has mechanical type work-bore switching mechanism without electric control system between the conventional master cylinder and the conventional vacuum type booster, it combines enough braking effect during a vacuum failure condition with the secure brake feel during the normal condition. This master cylinder operates in large bore during the normal operation and enables secure brake feel. It operates in smaller bore during a vacuum failure condition and enhances braking effect. The work-bore switching unit has a sealing pressure chamber of a large diameter, the work-bore switching action is carried out by open/close operation of the bore switching valve which has built-in this unit.