Master Cylinder

Normal braking 

Fig 1: Identifying Normal Braking

Courtesy of VOLVO CARS CORPORATION

The brake pedal is pressed and the piston nearest the servo cylinder moves to the left. Pressure builds up in the primary circuit.

This pressure also moves the second piston to the left and pressure builds up in the secondary circuit.

Leakage in the primary circuit 

Fig 2: Identifying Leakage In The Primary Circuit

Courtesy of VOLVO CARS CORPORATION

The brake pedal is pressed and the piston nearest the servo cylinder is moved, but does not build up pressure due to damage in the primary circuit. The piston moves until it comes into contact with the piston in the secondary circuit. Only then can pressure build up in the secondary circuit. The brake pedal stroke is longer than during normal braking.

Leakage in the secondary circuit 

Fig 3: Identifying Leakage In The Secondary Circuit

Courtesy of VOLVO CARS CORPORATION

The brake pedal is pressed and fluid in the primary circuit is pressed against the secondary circuit piston. The piston cannot build up pressure due to damage in the secondary circuit. It therefore moves to the left until it reaches the limit position in the master cylinder. Only then can pressure build up in the primary circuit.

The brake pedal stroke is longer than during normal braking.