is disposed within the float member 27 for actuating the reed switch 25 in the stepped projection 21 the downward movement of the float member 27 is limited by a stopper secured to the lowermost portion of the stepped projection 21 the diaphragm member 30 is made of a material having sufficient sealing characteristics and flexibility such as synthetic rubber the diaphragm member 30 has an annular brim 30a detachably but fluid tightly secured to the open end of the casing 10 an extendable or bendable portion 31 formed contiguous to and integral with the brim 30a and an annular valve body section 32 at the central portion of the diaphragm member 30 as is shown in fig2 and 4 the valve body section 32 includes an annular support 33 of substantially circular shape in section having a predetermined diameter and an annular lip 34 projecting upwardly and inwardly from the upper side of the support 33 the annular support 33 is provided with a plurality of in this embodiment eight vertical recesses 35 as is clearly shown in fig4 and 6 within the casing 10 as assembled with the cap member 20 and diaphragm member 30 an upper chamber r 1 is defined by the diaphragm member 30 between the under side of the cap member 20 and the upper side of the diaphragm member 30 and a lower chamber r 2 is also defined by the diaphragm member 30 between the under side of the diaphragm member 30 and the liquid level of the brake fluid stored in the casing 10 the upper chamber r 1 is in communication with the.