Line lock or roll control orginally designed for racing (burnouts or starting line hold) which is why it was put on the front brakes. Dirt buggies used them for parking brakes on the rear wheels. My Pantera has aftermarket Wilwoods which have no E brake provisions so I have hydraulic locks on the rear calipers. They are basically a pressure lock valve. The racing line-locs (I believe a Hurst item) are an electric solenoid valve which is button operated to release the brakes when the car is launched. Hall Pantera included a lever operated valve with their brake kits which I believe were Wilwood Superlights that I have on my car.
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