A brake master cylinder includes a BLS module to control the switching on of a brake lamp according to an operation of a piston provided to reciprocate within a cylinder body. The BLS module includes: a magnet fixed to a guide hole formed in the cylinder body in a lengthwise direction of the cylinder body; an actuating member coupled to the piston so as to be moved together with the piston as the piston is moved; a Hall sensor installed in the cylinder body at a location corresponding to the magnet for detect the magnetic flux density of the magnet; and a shielding member slidably provided in the guide hole and moved by the actuating member for shielding the Hall sensor from the magnet.