On-highway heavy trucks are using air disc brakes with increasing frequency. Disc brakes and traditional air drum brakes have different characteristics when it comes to brake adjustment and heat dissipation. These differences lead to different failure modes when overheated. This paper describes how adjustment affects the general braking capability of on-highway trucks fitted with disc and drum brakes. A loaded truck on a long, down-hill grade is used to illustrate how brake temperature increases over time, and how that temperature increase can result in a runaway condition. A “virtual” truck fitted with both traditional drum brakes and new disc brakes is used to illustrate operational differences between the two brake types.