Abstract: 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).