AIR DETECTOR AND AIR DETECTING METHOD

An air detector detects the presence or absence of air in a hydraulic braking system including a rearward-hydraulic-pressure controller 28 connected to a rearward chamber 66 located behind a pressurizing piston 34 in a master cylinder 26. When an instruction for detecting the presence or absence of air is output, a hydraulic pressure is accumulated in an accumulator 108 at S1. Pressure holding valves 170 are closed at S2. The presence or absence of air in an upstream portion 180 including the rearward-hydraulic-pressure controller 28 is detected at S3 based on a relationship between a servo pressure and a time of controlling an input chamber 122. The pressure holding valves 170 are opened at S6. The presence or absence of air of a downstream portion 182 including a pressure chamber 42 is detected at S7 based on a relationship between a stroke of a brake pedal 24 and the servo pressure. With these processings, the presence or absence of air is detected for the upstream portion and the downstream portion, separately. Thus, when air is detected, it is possible to identify a portion at which the air exists.