Showing how fluid fills the main part of the cylinder via the bypass port and the space between the two seals via the Intake port. This and the following drawings have been adapted from the dual-circuit system described in How Stuff Works.
On depressing the brake pedal the primary seal must pass the bypass port for pressure to be developed in the lines, and on return must clear the bypass hole to allow fluid expansion and contraction to pass in and out of the reservoir. If this doesn't happen - for example if the mechanical brake-light switch is screwed in too far - and the fluid expands, the brakes will be applied without the pedal being operated.
The Intake and Bypass ports in an MGB brake master.
The Bypass port is tiny.