The braking system on a Toyota Prius 2004-2009 is unlike a conventional system because the Brake Control Module (BCM) controls the brakes rather than the BCM adjusting the pressure created by the driver. This is so that co-operation between the Hybrid Control Module (HCM) and BCM can take place. When the brake pedal is pressed, information between the HCM and the BCM is negotiated to meet the driver's expectations. The result is a combination of regenerative braking and application of the hydraulic system.

How the system works

The HCM looks conventional but, under normal operation, there's no hydraulic connection between the master cylinder and the brake calipers, as the pressure applied to the brakes is generated by a hydraulic pump. There are sensors in the brake hydraulic module which monitor the pressure at the master cylinder and at the calipers. Any anomaly between these sensors will cause fault codes to be registered.

When ignition is switched on, the master cylinder isolation valves are closed, the stroke sensor solenoid valve opens and the rear brake linear valves V8 and V10 are closed. When the brake is applied, the BCM receives data from the master cylinder stroke sensor, about speed and position, and from the master cylinder pressure sensors 1 and 2. The stroke simulator piston moves under pressure to give the same sort of feel to the driver as he'd expect from a conventional system and to allow the master cylinder to stroke. This information is evaluated by the BCM and a braking strategy is decided.

Under low braking effort, above 10 km/h, there may be no hydraulic pressure applied to the calipers. All the braking may be done by regeneration (the drag of the large motor/generator in the transmission as it charges the battery). Greater braking demand is shared between the regeneration and hydraulic brake application. Communication between the Hybrid ECU and the BCM is via the CAN Bus.

Hydraulic brake application is achieved by controlling the linear valves. V3 to V6 are pressure-apply valves and V7 to V10 are pressure release-valves. The linear valves are not on/off valves, they're controlled by a Pulse Width Modulated (PWM) current which gives precise control of pressure in the calipers. Opening the apply valves allows pressure to build in the calipers. Opening the release valve allows the pressure to vent to the master cylinder. The pressure in the caliper and wheel speed is monitored and compared with data in the BCM. This allows the braking effect to be adjusted to meet the driver's demands. The system is also used to control brake application for traction control and vehicle stability control without the application of the brake.

Potential problems for the technician

Firstly, don't worry about high voltage - the system's not connected to the high voltage system.

Brake pad replacement on the Prius doesn't require any special tools but fault codes 'C134x Hydraulic System xxx Malfunction' may be

generated when the brake pedal is pressed for the first time, to bring the brake pads into contact with the disc. This is because there will be no significant pressure rise in the caliper, as would be expected by the BCM. The codes can be cleared with a scan tool or manually at the EOBD socket. The hydraulics can be bled without any special tools but this is best done with the assistance of a scan tool. If the hydraulic module is removed or replaced, it will need to be initialised so the system can determine the characteristics of the linear valves. For this you will need a suitable scan tool such as the G-Scan, available from Blue Print.

Bleeding the system

When the ignition is turned off, the valves in the hydraulic module return to their default position. The master cylinder cut off solenoids will then allow brake fluid to pass to the front brakes. To ensure this position and that no current can pass to the hydraulic pump, the two ABS relays are removed. The front brakes are bled by pumping the master cylinder to build up a pressure in the system and then releasing the bleed nipple to vent fluid. Doing this ensures the pipes running to and from the master cylinder and the stroke simulator are bled properly.

The rear brakes can be bled after replacing the two relays and turning on the ignition. The G-Scan will help here. Select 'bleed rear left' or 'bleed rear right' from the menu and follow the instructions. Alternatively, the rear calipers can be bled using accumulator pressure but great care must be taken and you will raise fault codes which will have to be cleared. Details of this process are covered in the Blue Print Hybrid course.

Brake pad replacement is a simple affair. When the relays are removed, a clear passage from the rear calipers to the reservoir is created and from the front calipers to the master cylinder. Toyota doesn't recommend clamping the brake hose and venting the caliper at the bleed nipple during piston retraction.

Clearing the codes

ABS/VSC codes cannot be cleared by removing the battery negative, for this you need a scan tool. Alternatively, ABS codes can be retrieved as blink codes and cleared at the OBD2 socket. A Special Service Tool, SST (a fancy name for a bridging wire), is inserted between pins 4 and 13 on the OBD2 socket. When the ignition is switched on, the ABS/ VSC light will flash. If the brake pedal is then pressed 8 times in 5 seconds, followed by the SST being removed, all ABS/VSC codes will be erased.