OK, an update. Not going well. The seal that the customer saw that was damaged was just the dust seal, not the oil seal. So that wasn't causing the leak. Oil seal had to be the problem, though I could not see any damage to the seal at all. Not sure why it was leaking, but I was still thinking I may have done something to it during the assembly process. I ended up buying another Hydro-Gear seal kit, because I couldn't find a seal the right size (1-13/16" OD x 1-3/8" ID x 3/16" HT). It's a single lip seal with no spring (not really room for one, the way it's made; more of a "solid" construction), and rubber coats the outer diameter of the seal as well. I looked online at a bunch of places that boasted they had the right seal for any application. They apparently forgot about this one, and so did McMaster-Carr. I'm starting to see maybe why this seal kit is so expensive. Must be a custom size. Since every measurement in the parts breakdown is metric, it's probably a metric seal. I looked for these too, but everything was either too big or too small. All the outer diameters I found jumped from 45mm to 47mm, when 46mm is what would work here. Aggravating!
So I rebuild it again with the 2nd seal kit. I was particularly careful not to cut the seal with the shaft keyway, wrapping plastic around the shaft as I put it through the seal. I could see a couple of shiny rings on the shaft, where I suppose the oil and dust seals were. But I couldn't feel the rings at all, so I thought all was OK. I gave the re-assembled motor to the customer the 2nd time yesterday, and he calls me today to tell me that he drove the mower about 10 feet and hydraulic fluid was leaking from the motor again already.
He's going to bring the whole mower this time (In case I didn't mention it earlier, it's a Ferris; not sure about the model number), which I'm glad for. That way I can see for myself how it leaks. I have no idea what I could've done wrong. I followed the steps in the Hydro-Gear service manual. I've rebuilt other wheel motors successfully. I'm going to contact Hydro-Gear Monday to see if they can offer any ideas. Did they get a bad batch of seals? Has the seal kit been sitting in a warehouse for years, allowing the rubber to harden or something? I'm afraid the only thing the customer is going to be able to do to fix the problem is replace the whole wheel motor if we can't stop the leaks on this one. And they're around $1500. Thanks, Hydro-Gear. Great job.