During a static load test with the pit valve closed, if the car sinks 6 inches in 15 minutes, what is the likely cause?

When conducting a static load test with the pit valve closed and observing that the car sinks 6 inches over a period of 15 minutes, this indicates a gradual loss of hydraulic pressure. This scenario most commonly points to a cylinder leak.

In a hydraulic system, the cylinder is responsible for supporting the weight of the car, and if there's a leak within the cylinder seals or elsewhere in the system, hydraulic fluid can escape. This loss of fluid would decrease the pressure required to maintain the car's position, resulting in the observed sinking. The closed pit valve, which prevents fluid from exiting or entering the system, further emphasizes that the loss of height of the car is due to internal issues rather than external influences.

Alternative options like an overloaded cylinder or a malfunctioning control system could lead to operational issues, but they would not typically cause a slow, gradual sink when the system is expected to hold steady. Similarly, a pit valve malfunction would result in immediate issues related to fluid flow rather than a slow drop in height. Therefore, the most logical explanation for the situation described is a leak in the cylinder.