Maybe some hard rain on some trash sitting around the fillercap? Dont know the Deere combines, would they have a water cooled hydraulic cooler? are you loosing coolant? have you used a pressurecleaner around the fillercap/breather?
For sure it would be nice to know where the water is comming from. Otherwise you waste a lot of time/oil/$ and it is a nogo for the hydraulic system on the long haul.
Take the time to investigate the problem...