the bolt sets the piston depth in the caliper, you want to run that in so the pads are as close to the rotor as possible, hopefully avoiding drag on the rotors while running with no brakes applied. That particular type is finicky. I just went through mine and to get the parking brake to work well, i end up with slight brake drag while running. these calipers don't seem to "float" back to the retracted position as well as they should.
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