Answer: A little trick is to remember that air is highly compressible. Have someone firmly hold down the brake pedal while the engine is running. Watch the master cylinder reservoir and have your partner release the pedal. If there is air in the system, a large spout of fluid will come out of the vent port. If no air is present, very little fluid will release because the air-free circuit will simply release the amount of fluid required to apply the brakes.
