Except Viggen
Master Cylinder
The master cylinder consists of a tandem cylinder made of aluminum.
As the car has two independent brake circuits, it also has one master cylinder with two independent pressure chambers. There are two pistons in the cylinder: one primary piston (closest to the brake pedal) and one secondary piston, each with a return spring. The secondary piston has a central valve of steel that opens a passage to the brake fluid reservoir in rest position.
From the master cylinder, two pipes (primary/secondary) run to the ABS unit (valve block) that, in turn, provides brake fluid to the wheel cylinders.
The brake fluid reservoir is mounted directly on the master cylinder.
Rest (brakes off) position
Master cylinder, rest position
No pressure
In rest position, the return springs press the pistons to their rear end positions. In this position, both return passages are open and the brake system is not under pressure. The backward movement is limited by a stop pin.
Brake applied (both circuits operating)
Master cylinder, brake applied (both circuits in operating)
No pressure
Servo pressure
When the brake pedal is depressed, the primary piston is pressed forward by the pushrod.
The return passage to the brake fluid reservoir closes and the pressure in front of the primary piston increases. The pressure also affects the secondary piston, which is pushed forward and its return passage is also closed by the central valve. The hydraulic pressure in the two brake circuits increases and, since the area of the pistons are the same, the pressure will be the same in both brake circuits. The pressure propagates throughout the brake system and affects the brake piston in each brake caliper. The brake pistons press the brake pads against the brake discs.
When the pedal is released, the pistons in the master cylinder are returned, opening the return passages. The pressure ceases and the piston in each brake cylinder takes up its rest position with the aid of the piston ring.
Brake applied (one circuit inoperative)
Main cylinder, brake applied (one circuit inoperative)
When the brake pedal is depressed and a leak has occurred in the primary circuit, for example, the primary piston is pressed forward by the pushrod until it acts mechanically on the secondary piston. The secondary piston closes the return passage with the central valve and hydraulic pressure builds up in the secondary circuit only.
If their is a leak in the secondary circuit, the secondary piston is pushed forward and will bottom in the master cylinder. In both these cases, a longer pedal travel will be required to obtain the desired braking power.
Front brake caliper
Each brake caliper consists of a hydraulic body with brake piston and two brake pads in a carrier. The front hydraulic body slides on two guide pins mounted on the carrier. The guide pins are manufactured in stainless steel and should not be lubricated.
There is one version of the hydraulic body, with a piston with two recesses to give a smaller area of contact with the brake pad and improved piston cooling. This design transfers only a small amount of heat to the brake fluid. The brakes are therefore able to withstand high thermal loads. However, it is important that the brake piston is fitted correctly after being dismantled so that correct cooling is attained.
When the brakes are applied, the hydraulic pressure acts partly on the brake piston directly and partly on the walls of the brake cylinder. The brake piston presses the inner brake pad against the brake disc. At the same time, the hydraulic body slides to the side and presses the outer brake pad against the brake disc as well. When the hydraulic pressure ceases, the brake piston in each cylinder is returned to its rest position by the piston ring.
Rear brake caliper
Each brake caliper consists of a hydraulic body divided on two halves, held together with bolts. Each half has a brake cylinder. During braking, the two pistons press the brake pads directly against the brake disc. To counteract uneven wear on the brake pads, there is a recess in each piston. It is extremely important that the pistons are fitted correctly after being dismantled.
When the brake pedal is released, the pistons are returned to rest position by the piston rings.