A method for measuring the volume of hydraulic fluid in the secondary circuit of a braking system, especially in an accumulator, through the use of a Hall-effect magneto resistive device. A secondary hydraulic circuit includes an accumulator (11, 137), an electric motor (71, 101) driven hydraulic pump (73, 143), and appropriate valving (133) for supplying braking force hydraulic pressure from a master cylinder to one or more wheel cylinders. There is a secondary hydraulic circuit supply path for each wheel brake cylinder and each path includes a first normally open solenoid actuable valve (41, 43, 57, 59, 97, 149) providing a brake fluid path from the master cylinder to the wheel brake cylinder and a first normally closed solenoid actuable valve (61, 65, 67, 69, 139, 141) selectively opening during an anti-lock event to provide a path to bleed fluid from the associated wheel cylinder to the accumulator (11, 137). One or more accumulator volume sensors (37, 39, 145, 147) provide an indication of the volume of brake fluid in the accumulators and an electronic control unit (127) responds to sensed accumulator volume for selectively opening a first normally closed valve during normal vehicle braking to maintain a preferred volume of brake fluid in the accumulator (11, 137) such that either an optimum anti-lock event or a traction control event may immediately occur.
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