Alternate Brakes 
Alternate brake hydraulic system selection is automatic.
The alternate brake system is powered by the Left hydraulic system. If the right hydraulic system pressure is low, an Alternate Source Selector Valve automatically opens to allow the left hydraulic system automatically supplies pressure to the alternate brake system. Pushing a brake pedal then opens the respective Alternate Brake Metering Valve and sends hydraulic pressure through the Alternate Antiskid Valves to the brakes.
Brake Source
The BRAKE SOURCE light illuminates and the EICAS advisory message BRAKE SOURCE displays if both the normal and the alternate brake system pressures are low. The light extinguishes if the Reserve Brakes system is selected and pressure is available. If the BRAKE SOURCE Light remains illuminated, the Brake Accumulator is the only source of power to the brakes.
- Only accumulator pressure is available for braking. During landing rollout, apply steady, increasing brake pressure and hold to a full stop. Do not taxi.
[Prevents possible loss of braking due to accumulator depletion]
Anti-Skid Protection
The same antiskid controller and light are used by both the normal and alternate brake systems. The controller regulates pressure through each alternate antiskid valve to a set of laterally paired wheels based on eight separate wheel speed transducer inputs.
When a potential skid is detected, the controller signals the appropriate valve to reduce brake pressure. Since each valve controls pressure for two wheels, brake pressure is reduced to both wheels. In addition to skid protection, locked wheel, hydroplane and touchdown protection are provided.
When the alternate brake system is powered, the ANTISKID light monitors the alternate brake system.
