Why does my brake pedal sometimes feel like it sinks when I'm holding my car stopped at a light?

Vehicles with power brakes are equipped with a device called a brake booster that aids the driver with brake force application. The brake booster relies on engine vacuum to boost the amount of force applied to the brake pedal by the driver of the car. Change in brake pedal height is caused by an increase in engine vacuum as engine load decreases. Engine vacuum is the difference in air pressure between the engine’s intake manifold and the outside air pressure.

Engine vacuum is dependent on many different conditions, but a simple way to think about it is in terms of engine load. Engine vacuum is highest when all accessories (i.e. alternator, water pump, air conditioner) 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.

To give a practical example, 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 decreases and engine vacuum increases. 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.