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An electronically controlled boosted brake system including an isolation valve between a source of pressurized fluid and a boost valve for selectively restricting the flow of fluid from the source to the boost valve. Another feature of the brake system relates to an accumulator valve connected betwe

An electronically controlled boosted brake system including an isolation valve between a source of pressurized fluid and a boost valve for selectively restricting the flow of fluid from the source to the boost valve. Another feature of the brake system relates to an accumulator valve connected between the boost valve and the accumulator for controlling when the operating pressure of the accumulator is supplied to the boost valve. Yet another feature is a unique master cylinder design including at least one primary piston positioned within a housing in an overlapping relationship with portions of a pair of secondary pistons.

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1. A vehicle braking system for applying pressurized hydraulic brake fluid to a plurality of wheel brakes upon actuation of a brake pedal by a vehicle operator, the braking system comprising: a source of pressurized brake fluid; anda boost valve for controlling a pressure of brake fluid from the sou

1. A vehicle braking system for applying pressurized hydraulic brake fluid to a plurality of wheel brakes upon actuation of a brake pedal by a vehicle operator, the braking system comprising: a source of pressurized brake fluid; anda boost valve for controlling a pressure of brake fluid from the source of pressurized fluid to the wheel brakes as a function of the operation of the brake pedal;wherein the braking system includes at least one isolation valve between the source of pressurized brake fluid and the boost valve for selectively restricting the flow of fluid from the source to the boost valve in an unactuated state, and wherein the isolation valve is a solenoid actuated digital valve movable between an open position permitting the flow of fluid between the source of pressurized fluid and the boost valve, and a closed position preventing the flow of fluid between the source of pressurized fluid and the boost valve. 2. The system of claim 1, wherein the boost valve is configured as an electronically controlled spool valve, and wherein the isolation valve is operable to reduce leakage of brake fluid through the spool valve when the spool valve is unactuated. 3. The system of claim 1, wherein the source of pressurized brake fluid includes at least one pump and an accumulator. 4. A vehicle braking system for applying pressurized hydraulic brake fluid to a plurality of wheel brakes upon actuation of a brake pedal by a vehicle operator or upon an automatic braking event, the braking system comprising: a source of pressurized brake fluid, the source of pressurized brake fluid including at least one motor driven pump and an accumulator, the accumulator operable to supply fluid at an operating pressure; anda boost valve connected to receive fluid at an input pressure from both the pump and the accumulator, and operable for controlling a pressure of brake fluid to the wheel brakes as a function of the operation of the brake pedal or the automatic braking event;wherein the system further includes an accumulator valve connected between the boost valve and the accumulator for controlling when the operating pressure of the accumulator is supplied to the boost valve, and wherein the accumulator valve selectively restricts the flow of fluid between the accumulator and both the pump and the boost valve when the input pressure required by the boost valve exceeds the operating pressure of the accumulator. 5. The system of claim 4, wherein the accumulator is a medium pressure accumulator providing pressurized braking fluid to the boost valve at pressure levels substantially supporting braking demands below a predetermined pressure, and wherein the pump provides pressurized braking fluid to substantially support braking demands above the predetermined pressure. 6. The system of claim 5, wherein the medium pressure accumulator vents fluid to a fluid reservoir when the pressure within the medium pressure accumulator rises above the predetermined pressure. 7. The system of claim 4, wherein the accumulator valve is a pilot operated valve movable between open, closed, and metering positions. 8. The system of claim 7, wherein the pilot operated valve references the pressure at the source of pressurized fluid and the pressure at an outlet of the boost valve between the boost valve and the wheel brakes. 9. A vehicle braking system for applying pressurized hydraulic brake fluid to a plurality of wheel brakes upon actuation of a brake pedal by a vehicle operator or upon an automatic braking event, the braking system comprising: a source of pressurized brake fluid, the source of pressurized brake fluid including at least one motor driven pump and an accumulator, the accumulator operable to supply fluid at an operating pressure; anda boost valve connected to receive fluid at an input pressure from both the pump and the accumulator, and operable for controlling a pressure of brake fluid to the wheel brakes as a function of the operation of the brake pedal or the automatic braking event;wherein the system further includes an accumulator valve connected between the boost valve and the accumulator for controlling when the operating pressure of the accumulator is supplied to the boost valve, wherein the accumulator valve is a solenoid actuated digital valve movable between an open position permitting the flow of fluid between the pump and the accumulator, and a closed position preventing the flow of fluid between the pump and the accumulator.