The vacuum available at an engine's intake manifold generated by the engine's pumping action. Used as the assist force on most power brakes.
As the pistons move down on the intake stroke, they create a suction or vacuum in the intake manifold. This vacuum reading can be used to determine how well the engine is running.
Low air pressure in the intake manifold of a running engine, caused by the descending pistons creating empty space in the cylinders faster than the entering air can fill it.