Freightliner M2 106 water in power steering fluid

Started with this fleet in October last year, as the only mechanic and with no previous service records. the trucks are 2008 M2s with MBE900s. Airport service only so no road water spray, and only one wash all winter.

This winter I was going through the trucks and found one of them had very water contaminated power steering fluid. I changed the fluid, flushed it by filling the tank, and running the engine with the return hose in a bucket until the fluid coming out was all new, and installed a new filter and fluid. I was checking them again today and the same truck was very contaminated again. Both times not to the point where the reservoir was overfilled, the level seemed fine. Any ideas where this contamination might be coming from? Or was it just contaminated once and I needed to turn the steering a couple times when flushing?