The purpose of the Accumulator on a Hydraulic Assist System is ______.

The purpose of the Accumulator on a Hydraulic Assist System is to store fluid pressure for assist in case the hydraulic supply is lost. In hydraulic assist systems, accumulators serve as a backup reservoir that maintains a certain level of hydraulic pressure. This ensures that, if the primary hydraulic supply fails or is interrupted, there is still sufficient pressure available to continue operating essential functions such as brakes or steering. This safety feature is crucial to maintaining the vehicle's performance and safety under unexpected conditions.

In the context of the other options, increasing "boost" to the brake pressure when towing refers to an enhancement feature primarily associated with brake systems but does not capture the primary role of an accumulator. Providing assist pressure to the power steering system during rapid steering actions is not the main function of an accumulator, as it specifically deals with emergency fluid storage rather than responsiveness during maneuvering. Lastly, the function of expanding wheel cylinder pistons to operate brake shoes more quickly is related to brake actuation rather than the storage of fluid pressure. Thus, the role of the accumulator is clearly centered around ensuring reliability and security in hydraulic functionality.