A master cylinder assembly for a bicycle disc brake having a housing defining a cylinder, a piston received in the cylinder, a lever pivotably attached to the housing, and an adjustment guide body that is operatively associated with the piston. The piston is moveable within the cylinder between a select starting position and a pressurized position. The lever is operatively associated with the piston to move the piston from the select starting position to the pressurized position as the lever is pivoted between a rest position and a fully-actuated position. The adjustment guide body is pivotable between a first position and a second position for adjusting the select starting position of the piston.