A brake system for a vehicle may include a primary braking system, a backup braking system, and a braking system controller. The primary braking system may apply braking energy to wheels of the vehicle. The backup braking system may include a master cylinder configured to supply braking energy to one or more of the wheels of the vehicle. The braking system controller may be configured to select a mode of backup braking based on a functional state of the primary braking system. The braking system controller may control the master cylinder to supply braking energy to less than all of the wheels of the vehicle in a first mode of backup braking and supply the braking energy to all of the wheels of the vehicle in a second mode of backup braking. A parking brake may assist the backup braking system by utilizing proportional braking in a backup braking mode.