Final answer:
The maximum straight-line braking is achieved by using the front and rear brakes at the same time without locking either wheel.
Step-by-step explanation:
The maximum straight-line braking is achieved by using the front and rear brakes at the same time without locking either wheel. When both brakes are applied simultaneously, the weight of the vehicle is distributed more evenly, allowing for greater stopping power. This method ensures that the front brake provides most of the stopping force, while the rear brake helps to stabilize the vehicle and prevent it from skidding.