System Overview - Saab 1992 900 L4-2118cc 2.1L DOHC Service Manual

System Overview

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When the brake pedal is depressed, pedal pressure assisted by power from the servo unit pushes the input rod into the master cylinder. The input rod pushes the primary plunger up the bore which closes the cut-off port to the fluid reservoir. This creates pressure ahead of the primary plunger. This pressure also acts on the secondary plunger, pushing it up the bore and closing the cut-off port from the secondary chamber. The pressure in both circuits increases and, because the plungers are the same size, pressure in each circuit is the same. Fluid pressure is directed to the brake system, which advances the piston in each individual caliper. When the pedal is released, the plungers in the master cylinder retract and cut-off ports are opened. The pressure is exhausted and the piston seal in each caliper returns the piston to its retracted position.
If there is a fluid leak in the primary circuit, the input rod pushes the primary plunger up the bore until it acts mechanically on the secondary plunger. The secondary plunger closes the cut-off port, allowing hydraulic pressure to build up in the secondary circuit. If a leak should occur in the secondary circuit, the secondary plunger will be pushed right up the bore until it reaches the end of the cylinder. In either case, greater pedal movement will be required to achieve desired braking effect.