when you hook the air lines up to tow a M939, don't drop the parking brake lever. Push the black "spring brake overide" button under the dash next to your right knee as your sitting in the driver seat. That should release the "spring brakes" but allow the regular brakes to engage when you use your's. Worse come to worse, get some caging bolts, disconnect the air lines, and disable the rear brakes, the front axle doesn't have spring brakes. but then you'll only have the primary mover's brakes to slow things down. When doing that, you're limited to 25 MPH on road and 15 MPH off road from what I remember.