Hydraulic Assist Systems are typically used when ______.

Hydraulic Assist Systems are primarily utilized in situations where there is insufficient engine vacuum to operate a vacuum booster. These systems provide increased braking force by assisting the brake pedal effort, which can be particularly beneficial in modern vehicles where the engine's ability to generate vacuum may be compromised due to various factors such as engine design, load conditions, or modifications.

When the engine vacuum is inadequate, relying solely on vacuum boosters can lead to decreased braking efficiency and compromised safety. Hydraulic assists counteract this issue by leveraging hydraulic pressure to enhance braking performance, ensuring that the driver maintains control and effectiveness in stopping the vehicle, especially under challenging conditions.

The other options, while related to performance issues in vehicles, do not typically dictate the need for hydraulic assist systems in the same straightforward manner. Thus, the context of the correct answer aligns specifically with scenarios that involve vacuum supply concerns, making it the most relevant choice in this case.