What safety feature prevents a hydraulic elevator piston from descending?

The check valve is a critical safety feature in hydraulic elevators that serves to prevent the elevator piston from descending unintentionally. This device allows fluid to flow in one direction—into the piston when the elevator ascends—while preventing flow in the opposite direction when the system is under pressure. Essentially, if there is a loss of hydraulic pressure, for instance, due to a rupture or failure in the system, the check valve will seal off the fluid, keeping the piston in place and thus preventing a dangerous fall of the elevator.

This function is essential for ensuring passenger safety, as it mitigates the risk associated with potential failures that may lead to uncontrolled descent. Moreover, it contributes to the overall reliability and performance of the hydraulic elevator system.

In contrast, while features like the overload valve, emergency brake, and pressure switch have important roles in overall elevator safety, they address different aspects. The overload valve protects against excessive loads, the emergency brake provides a mechanical stop in case of system failure, and the pressure switch monitors fluid pressure but does not prevent descent directly. Therefore, the check valve's unique function of ensuring the piston remains stationary under pressure loss is what makes it the correct answer.