Why does my brake pedal sometimes feel like it sinks when I'm holding my car stopped at a light?
This change in brake pedal height is caused by an increase in engine vacuum. The "power brakes" system in most modern vehicles uses engine vacuum to boost the amount of force applied to the brake pedal by the driver of the car. The device that does this, called the brake booster, makes it easier for the driver to stop the vehicle.
Engine vacuum is affected by many different conditions. Engine vacuum is highest when all accessories are off and the accelerator pedal is released (low load), and lowest when all accessories are turned on and the accelerator pedal is held to the floor (high load). Any change in engine vacuum (load) can affect the amount of assist or boost, provided by the brake booster.
One system that applies a noticeable load to the engine is the air conditioning. During normal operation, the air conditioning system constantly cycles on and off, changing the amount of load on the engine. When the air conditioning cycles off, the load on the engine is decreased and the amount of engine vacuum is increased. If the driver is applying the brake pedal when the air conditioning cycles off, the increase in engine vacuum increases the amount of boost applied to the brake pedal. That increase is felt as a slight drop in brake pedal height and is a normal characteristic.