I installed cruise control on the 3126, I wired in a service brake input using a change over relay, and I had to figure out the cruise control set resume polarity in relation to switch and ecm. If you have CAT ET, you can live view what position the set/resume is. On my truck when the cruise control is armed I can set my speed and increase or decrease the speed using the set/res switch. Hitting the brake disengages the cruise control but it remains armed, pac brake is also inhibited when cruise control is armed. Since yours is a C7 the logic might be different.
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