613c speed reducer leaking oil.

Got a 613c speed reducer on an elevator that's leaking oil out the breather. I've already replaced the lip seals, wear shims, and pressure relief valve, but it's still leaking while running. Any ideas what I might be missing?

Is oil maybe leaking past the hydraulic motor seal and getting into the speed reducer? If it's leaking that badly, could the case drain hose on the motor be too slow to get the oil back to the reservoir?

The motor's fine. It seems to be building pressure when it heats up. The first 10-15 minutes are dry, then it starts leaking. When I pulled the fluid level plug, I could feel and hear pressure coming out. The relief valve is new and moves freely. I noticed the oil was really aerated and foamy, so I switched to a different oil. I'm also thinking about adding another breather to help.

Did you end up trying a different oil?