Never had one, or worked on one, but it seems a stretch to point at the brake switch. To eliminate it, look under the dash, or on the firewall. There should be two, a NO for the brake lights, and a NC for the ECM. Fill the air tanks, disconnect the wires, and apply the brakes. Use a multimeter to verify operation of each switch, paying attention to the NC. More likely, the clutch needs adjusting. I'm basing this on my understanding of auto shift transmissions. The ECM uses engine RPM, road speed, throttle position, boost pressure, and a handful of other parameters to pick a gear position. Nowhere in the list is brake switch.