Patent ID: 7018002

Claim:
A master cylinder ( 1 ) for a motor vehicle braking system, comprising: a body ( 2 ) made of non-magnetic material; a bore ( 3 ) located in the body ( 2 ) of the master cylinder; a variable-volume pressure chamber ( 5 ) within the bore ( 3 ); a piston ( 34 ) located in the bore that slides therein for varying the volume of the pressure chamber ( 5 ); and a detection device ( 9 ) fixed to the body ( 2 ) of the master cylinder and facing the bore for detecting actuation of the braking system corresponding to movement of the piston, said detection device having first circuit and a second circuit that are opened or closed by a magnetic piece ( 19 ) carried by the piston ( 4 ), characterized in that said first magnetic circuit is defined by a first pole piece ( 29 ) having a first gap ( 26 ) formed with the magnetic piece ( 19 ), a second pole piece ( 30 ) having a second gap ( 27 ) formed with the magnetic piece ( 19 ) and a magnet ( 32 ) located between the first and second pole pieces such that when said piston is in a rest position a residual magnetic flux flows in said first magnetic circuit by way of said first and second air gaps and said second magnetic circuit is defined by a third pole piece ( 31 ) having a third gap ( 25 ) formed with the magnetic piece ( 19 ) and a fourth gap ( 24 ) formed between said first pole piece ( 29 ) and said third pole piece ( 31 ) and a magnetic-field-variation sensor ( 28 ) that is isolated from said residual magnetic flux flow in said first magnetic circuit by said third gap and said fourth gap when the piston ( 34 ) is in the rest position and only when the piston moves is the magnetic-field-variation sensor ( 28 ) closed and magnetic flux flow occurs in the second magnetic circuit to provide an indication of the movement of the piston ( 34 ).