PURPOSE:To simplify the construction by providing a pressure switch, which is to sense any generation of pressure difference between the low and high air chambers, and by allowing an alarm to be actuated after a certain while has passed in the event of no generation of pressure difference between air chambers during brake application. CONSTITUTION:At a power piston 4 in a booster 2, a normally closed pressure switch 14 is installed in such an arragement as turned off when a pressure difference is generated between the low and high air chambers 5, 6, and a trouble sensing circuit is formed including this pressure switch 14. In this circuit, a transistor Q is as switching element connected with a circuit, which is in connection from the power supply E to the alarm lamp 16 through a brake switch 15, and the transistor Q is actuated when an operating signal is fed through a pressure switch 14 and a time lag circuit 17. Thus the alarm lamp 16 will light up.
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