Almost always excess case pressure rather than a faulty seal. Shaft seals are rated for modest case pressure, and a restricted, undersized or shared case drain line pushes pressure above that and forces the seal out. Repeat failures should be investigated as a case drain problem, not a seal quality problem.

Directly back to tank, full size, with no restriction, no check valve and preferably not shared with another component. It should terminate below the fluid level to keep the case full. These requirements are frequently ignored on retrofits, which is why shaft seal failures cluster on modified machines.

No - the wearing elements are specific to the geometry. Gerotor sets for orbital motors, vane and cartridge kits for vane motors, piston and valve plate sets for piston motors, gear and wear plate sets for gear motors. Bearings and shaft assemblies are the parts most likely to be common.

The closest clearance in that motor type - the valve plate and slippers on a piston motor, the gerotor element on an orbital, the vanes and ring on a vane motor. In every case the symptom is the same: lost speed and torque as internal leakage rises, with no dramatic failure event.

Whenever there is external side load and the motor is opened, and always if there is any play or roughness. Bearings carry any radial load from a sprocket, pulley or wheel, and a worn bearing lets the shaft deflect, which then destroys the shaft seal and the internal clearances in turn.

Usually for larger and piston-type motors, where the housing is the expensive part and the wearing elements are all renewable. Small gear and orbital motors are often cheaper to replace outright. Either way, the system contamination that caused the wear must be addressed or the rebuild has a short life.

Flanges, adapters and couplings matched to the shaft and to the load type. The critical distinction is whether the motor will see radial load - a shaft-mount motor used where a wheel-mount unit with integral bearings is needed will fail at the bearing regardless of the hydraulic specification.