The M & G unit requires placing part of the system in-line with the brake booster, and often times, depending on the tow car, there is insufficient space to allow it's installation.

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The U.S. Gear system involves placing a pully on the brake pedal. An installation picture is attached.
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The units we have installed by far the most are Air Force One, for Diesel engine units with air brakes, or Stay-n-play Duo for gas rigs with hydraulic brakes. They offer break-away feature as part of the system (not an add-on option as with M & G). See pictures below.

afo_02.jpgafo_01.jpgafo.jpgafo_Vacuum-Diagram.jpg

Hope this information helps.