Brake System; Brake Booster - Lexus GS300 1998 Owner's Manual

STARTING AND DRIVING

BRAKE SYSTEM

This brake system has 2 independent hydraulic circuits. If
either circuit should fail, the other will still work. However,
the pedal will be harder to press, and your stopping distance
will be longer. Also, the brake system warning light may
come on.
CAUTION
Do not drive your vehicle with only a single brake system.
Have your brakes fixed immediately.

Brake booster

The brake booster uses brake fluid pressurized by the pump
to power–assist the brakes. If the brake booster fails during
driving, the brake system warning light comes on and buzzer
sounds continuously. In this case, the brakes may not work
properly. If they do not work well, depress the brake pedal
firmly. If the brake system warning light comes on,
immediately stop your vehicle and contact your Lexus dealer.
The brake system warning light may come on for about 60
seconds after the engine is started. It is normal if the light goes
out after a while.
Depressing the brake pedal repeatedly may turn on the brake
system warning light and buzzer. It is normal if the light goes
out and the buzzer stops sounding after a few seconds.
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You may hear a small sound in the engine compartment after
the engine is started or the brake pedal is depressed
repeatedly. This is a pump pulsating sound of the brake
system, and it is not a malfunction.