A master cylinder for a bicycle hydraulic disc brake includes a cylinder wall defining a cylinder interior and a cylinder exterior. A piston is received within the cylinder interior. A reservoir for hydraulic fluid is defined in part by a portion of the cylinder exterior, the portion of the cylinder exterior protruding into the reservoir. A port through the portion of the cylinder exterior which protrudes into the reservoir provides fluid communication between the reservoir and the cylinder interior. The port is located on the portion of the cylinder exterior which protrudes into the reservoir at a position which prevents air bubbles within the reservoir from entering the port.