Abstract: Three adjacent disks carried by an "S" cam shaft of a truck brake system sense motion of system components beyond predetermined limits to indicate need for adjustment and parts replacement and annunciate such information at a distance from the brake system. A first outer disk journaled on the "S" cam shaft is interconnected to the slack adjuster arm to rotate responsive to motion of the slack adjuster arm. A second medial disk journaled on the "S" cam shaft is supported by external structure to remain stationary relative to that shaft. A third outer disk is irrotatably carried by the "S" cam shaft to rotate with it. The second medial disk carries two Hall effect switches to sense the rotary position of magnets carried in the first and third adjacent outer disks to determine motion of the slack adjuster arm and the "S" cam shaft beyond predetermined limits to indicate need for brake adjustment and brake shoe replacement. Sensed data is transmitted to an annunciator at a distance from the braking system.