This valve is used in each out let fitting to the brake lines. Note that how the residual pressure valve allows fluid flow out of the master cylinder. However it resists free how of fluid back in to the cylinder and many master cylinders use these valves.
The rubber boot prevents dust, dirt and moisture from entering the hack of the master cylinder. Dual master cylinders are now used because they will still provide braking action when a major hydraulic leak develops with single-piston master cylinder, a leak could cause sudden and complete loss of brake.