I drove a 57 Dodge 1-ton wrecker several years ago that used a Mico LeverLock brake system. We used it, in addition to the parking brake, to lock all the wheels during winching jobs. Apply the brake pedal, FIRMLY, and hold it while moving the lever to the "locked" position. It stopped off the return line to the master cylinder, and trapped the fluid in the brake lines. On a dual-line master cylinder, I assume the valve would have needed two circuits. To release, just swing the lever to the "open" position. No, I didn't trust it for parking!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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