Brake system with fault-tolerant features

A brake system for actuating a plurality of wheel brakes comprising first and second pairs of wheel brakes, the system comprising:

• a reservoir;
• a motor-driven master cylinder operable during a backup braking mode by actuation of an electric motor of the master cylinder to generate brake actuating pressure at first and second MC outputs to hydraulically actuate the first and second pairs of wheel brakes, respectively;
• a power transmission unit configured to selectively provide pressurized hydraulic fluid to a PTU output to actuate the first and second pairs of wheel brakes in a normal non-failure braking mode, the power transmission unit including a side-mounted electric PTU motor configured to selectively pressurize the hydraulic fluid by transmitting rotary motion to a ball nut assembly of the power transmission unit via a force translator mechanically interposed between the electric PTU motor and the ball nut assembly;
• an electronic control unit configured to control the power transmission unit and the master cylinder;
• and
• first and second two-position three-way valves, each three-way valve being hydraulically connected with the master cylinder, the power transmission unit, and a selected pair of the first and second pairs of wheel brakes;
• each of the first and second three-way valves selectively controlling hydraulic fluid flow from a chosen one of the master cylinder and the power transmission unit to the selected pair of wheel brakes.