When Hydraulic Motor Case-Drains Are Optional—Unless You Want Reliability!
As you're hopefully well aware, an unrestricted case-drain line is essential on a piston-type hydraulic motor, whether it's axial, radial or bent-axis design. But when it comes to gear designs—external, internal and gerotor, a case-drain line is generally considered optional. This is because these types of motors don't have a large-volume case or sump like piston motors do. But they do have internal leakage which by design, usually accumulates in a small cavity just before the shaft seal.
In theory, by installing a high-pressure shaft seal and two check valves as shown in the picture below, internal leakage is drained into the return line (something that's not acceptable for a piston motor or pump). And this is why a dedicated drain line is usually considered optional.