Overview of hydraulic system

Fig 1: Identifying Brake System

Courtesy of VOLVO CARS OF NORTH AMERICA.

Divided brake system

The two-circuit brake system is also separated in the hydraulic unit. For this purpose the pump has two pistons. When ABS is active each piston supplies pressure to one of the two brake circuits, that is, primary and secondary brake circuits. The pump is driven by an electric motor.

For each brake circuit the system has two inlet and outlet valves. The inlet valve is a valve that normally is open while the outlet valve is a non-return valve that normally is closed. This makes it possible for the brake system to work conventionally when the brakes are affected.

The system has a low-choice control of the rear wheel brakes. This means that the brake pressure on the rear wheel brakes is mutually even and is adapted to the most critical wheel.

The rear wheels' brake circuit has no pressure reducing valves. The lower pressure that is needed is controlled by the EBD-system.

Functions

There are three situations:

keep the pressure by closing the inlet valve. The outlet valve is not supplied with current and thus remains closed.
lower the pressure by opening the outlet valve. Therefore both valves, inlet and outlet, are supplied with current.
build up pressure again by opening the inlet valve, at the same time as the outlet valve is closed. None of the valves are supplied with more current and return to the start position. The pump remains connected and builds up pressure in the wheel brake cylinder.

The pump is driven constantly during ABS-activation. The inlet and outlet valves control the brake pressure in the brake cylinders.

The pump is not operated when EBD is activated. The pressure in the rear brake calipers is controlled by the valves.

When ABS is activated the brake fluid circulates in the hydraulic unit's high-pressure part and is not directed back to the reservoir.

Buffer compartments

Buffer compartments' functions (S):

When the outlet valve opens the pressure in the wheel cylinder must be lowered very quickly. The purpose of the buffer compartments is to catch the fluid, especially if the pump has not yet reached full RPM. It is a small compartment that holds approx. 2 cm³ with a weak spring and piston. The high pressure can now be unloaded as the fluid can be pressed into the compartment.
The weak spring behind the piston helps the pump start if it starts before the outlet valve opens.

Buffer compartments' function (D):

These buffer compartments have the purpose of absorbing the pressure variations in the system caused by the piston pump. This helps to reduce the sound level in the system.