After installing a rebuilt hydraulic hand pump, it is found that the handle cannot be moved in the pumping direction. What is the most likely cause?

The scenario where the handle of a rebuilt hydraulic hand pump cannot be moved in the pumping direction typically suggests an issue with fluid flow or pressure within the system. In this case, the most likely cause is related to the installation of the check valves.

When the outlet check valve is incorrectly installed, it may prevent fluid from exiting the pump during the pumping action. This would create a pressure buildup within the pump during the effort to actuate the handle. As a result, the handle would feel locked or stuck due to the inability to displace fluid, leading to resistance in its movement.

In contrast, if the input check valve were incorrectly installed, it might lead to issues related to the pump sucking in fluid, but it wouldn't necessarily prevent the handle from being moved in the pumping direction. A blocked fluid reservoir could impede the flow as well, however, this situation would more commonly cause air or flow issues through the system and not specifically prevent handle movement due to pressure buildup. Worn pump seals would generally lead to leaks or reduced efficiency, rather than a complete halt in movement.

Thus, the scenario aligns closely with the consequences of an incorrectly installed outlet check valve which effectively blocks the fluid path, resulting in the handle being immovable.