Overheated brakes on a heavy truck can lead to a loss of braking and subsequent crash. This typically occurs after prolonged braking on a downhill grade. The problem is often accompanied by improper brake adjustment, causing one or more of the actuators to reach the end of travel due to thermal expansion. A thermodynamic model of a drum-type foundation brake exists and has been used successfully to study this phenomenon. However, newer on-highway trucks are often fitted with disc brakes. A similar thermodynamic model for a heavy truck disc brake does not exist, making the study of the problem described above difficult to address. This paper describes a new thermodynamic model of a disc foundation brake, such as those typically fitted on newer heavy trucks. The model is based on classic thermodynamic heat transfer equations that describe the rate of heat flow through the system as a function of time and location within the rotor and pad materials. Simulation results are used to compare the new disc brake model with the existing drum brake model.