My parents lived in Germany for 7 years and I worked there for a year in 1980. I had noticed that the M-B service agent attached a tag to the master cylinder of our car certifying that the fluid was changed and that was checked when my mother took the car in for the biennial TuV vehicle inspection. Not long after that a work colleague had the brakes fail on his VW bug due to fluid boiling after he drove with the hand brake partially on. Since then I've been aware of these potential issues and of-course have acquired equipment to efficiently change the fluid on my own conventional cars.
Almost every master cylinder is vented to atmosphere to allow the fluid level to fall as pads wear. A few examples (Delco-Remy '60s-'80s?) had a bellows to compensate for the volume change. The vent, where used, is often hard to spot, just a small razor slit or pinhole in the rubber gasket. Another contributor to air entry is that the fluid level "breaths" slightly when the pads go from the just-relaxed position to being bumped out a tiny bit more when the disks flex under wheel loading. In fact disk brake technology relies on that phenomenon to acquire pad clearance to the disk. Manual clutch hydraulics experience a similar effect at the throwout bearing due to crankshaft endfloat.
So, the brake fluid can absorb moisture both from what wanders through the vent and the slow exchange of that air due to the latter process. I have noticed how the fluid in the reservoir seems to deteriorate faster than what comes out of the calipers but I would expect that in-theory moisture should eventually be absorbed uniformly in a hygroscopic fluid.
The Kona's system (see second graphic) is perhaps three times more complicated than a conventional system, additionally containing an integrated second motor-driven master cylinder and a handful of electric and check valves to effect the combination of regen and braking. It's clear from an engineering perspective that there is a design balance required between using conventional, established, safety-proven systems to avoid getting sued, and the main goal of designing an efficient EV ... otherwise it might all be done electrically. As best as I have noted, every EV has a similar system, including GM's EV1 from the '90s, see first graphic below.
My dealer did my scheduled brake fluid change a few weeks ago at my second service and I'm fairly certain they used a pressure flusher and possibly an OBD routine to cycle the valves and pump fluid through. I noticed 12V battery activity throughout the service and they billed me for 1 litre, about what I would expect. I checked all the bleed nipples and it appeared all had been recently opened as they were wet. The fluid change is not optional in the EU schedule as it appears to be in the US one. That could be because an inspection using a proper instrument may allow for a change to be delayed if the moisture content is within spec, or simply allowing the legal right to ignore conventional maintenance without that affecting warranty obligations or rights.
The cost of damaged brake system parts in a Kona is something I would not want to find out just because I wanted to save a few dollars. Plus, if it's required to maintain the warranty I'm hardly going to to argue the point with a service advisor.
