A possible answer to the stepper motor 'slack upon power off' condition is to use a worm gear mechanism. That is, the stepper motor rotates a worm screw, which drives a right angle shaft that turns the brake adjustment valve. A mechanical advantage is built into the worm drive, producing more torque. In addition, once the stepper motor loses power, the output gear is locked in place. Reverse rotation of the output gear is caused by reversing the stepper motor rotation.
Here is one possible worm drive source.:
Worm Drive
Here is one possible worm drive source.:
Worm Drive
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