I noticed early in the operation of mine that the relief valve leaks past the set screw threads. ( I found it right away because I had the thing running at max speed and the 4th valve wasn't working so , the relief was " busy " all the time
Even now , after setting correctly , I still get a little leakage. I placed a towel under it to capture the oil. I think I ran 3 or 4 battery cycles before the towel was getting oil soaked.
Anyway, I bet that leakage is what got into the servo? It's the only possibility
Here's a pic, notice how the relief is directly above the servos?
Mario