Final answer:
The trollie brake should be used to park a combination vehicle on a grade, but not at loading docks.
Step-by-step explanation:
When parking a combination vehicle, it is important to use the trollie brake to park on a grade. The trollie brake, also known as the trailer brake, is used to hold the combination vehicle in place when parked on an incline or decline. By engaging the trollie brake, the driver ensures that the vehicle will not roll or drift while parked.
The trollie brake should not be used when parking at loading docks. In this scenario, the vehicle is typically not on a grade, and the brake system of the vehicle, including the parking brake, is sufficient to secure the vehicle in place.
In summary, the trollie brake should be used to park a combination vehicle on a grade, but not when parking at loading docks.