Final answer:
The main disadvantage of disc brake units on trucks during downhill runs is inefficient heat dissipation, which can result in brake fade or failure.
Step-by-step explanation:
The main disadvantage of disc brake units in the context of truck brakes, especially when controlling speed on a downhill run, is the challenge of heat dissipation. As the brakes convert gravitational potential energy into internal energy (higher temperature), this increase in temperature can become problematic. Given the large mass of the truck compared to the brake material, there might be an insufficient transfer of heat from the brakes to the environment. The rapid increase in temperature can lead to brake fade, where the efficiency of the brakes diminishes, or even brake failure due to the material's limitations in withstanding extreme temperatures.