Hydraulic System: Description and Operation

Hydraulic Brake Actuation

The hydraulic brake actuation system consists of the following components:

- Brake master cylinder
- Brake pedal and bracket assembly
- Brake tubes and hoses
- Hydraulic Control Unit (HCU)
- Pressure control valve (non-hybrid vehicles only)

The hydraulic brake system is diagonally split with the LH front and RH rear brakes on one circuit and the RH front and LH rear brakes on the other circuit.

Hydraulic Brake Actuation

The brake pedal is connected to the power brake booster, which is connected to the brake master cylinder. When the brake pedal is pressed, brake fluid is pushed from the master cylinder through the steel tubes, HCU, pressure control valve (non-hybrid vehicles only) and flexible hoses to the front brake calipers and rear wheel cylinders. When the brake pedal is released, brake fluid pressure is relieved, returning the front and rear brake components to the unapplied position.

Brake Pedal Assembly - Hybrid

Hybrid vehicles are equipped with a brake pedal assembly that utilizes driver input and electronic components to apply the brakes. The brake booster push rod has an elongated slot that attaches to the brake pedal with a clevis pin. The elongated slot allows for a small amount of pedal travel (freeplay) to occur without the brake pedal applying pressure on the booster push rod. This freeplay allows regenerative braking to occur. To compensate for the freeplay, the pedal assembly is equipped with a hydraulic brake pedal feel simulator that provides the same feel and resistance as a conventional brake pedal. On light or normal brake applications, sensors attached to the brake pedal assembly measure brake pedal angle and the amount of pressure applied to the brake pedal. The sensors send signals to ABS module which activates a solenoid inside the brake booster. The solenoid draws the booster push rod into the booster, applying pressure to the master cylinder piston which then applies hydraulic pressure to the brake system. On hard brake applications, the brake pedal will travel to the end of the elongated slot, eliminating the freeplay and physically engaging the booster push rod, operating the brakes similar to a conventional brake actuation system.