The mighty-vac is a blessing, I also embrace gravity bleeding where possible. One reason it could have been so bad is that brake fluid, the moment it is opened, begins absorbing atmospheric moisture and becomes contaminated if left open for any length of time. Years are not kind to it and it must be regularly flushed. I have seen flushing intervals often from 2 - 5 years. I do mine at 2 years regardless of mileage.
This is one of those things people systematically ignore because it costs money and contaminated fluid is often asymptomatic, at least initially. It's on par with the automatic transmission flushing, it gets ignored, much to the detriment of the owner that wants to enjoy the vehicle well into it's "classic-hood".
It all comes down to lack of maintenance causing premature failure of other components like calipers, master cyl, slave cyl et al. Once a new vehicle looses it's novelty, the owners often forget about shit like this and only fix things that make noise. It's not unusual for hydraulic components to last decades on a properly maintained vehicle, or fail within a handful of years if ignored, its a crap shoot. You got it right by flushing it completely and continue with the normal maintenance schedule and if your lucky you have some years left in the master and slave cylinders.
Since the PO likely established a pattern of ignoring maintenance, guess what else is a pain in the ass... forks. I would look there next.
Generally speaking, anytime I am looking at a vehicle to buy I ask for service records first (I learned this as a German car nut), its imperative and indicates not only what was done and what was not but also how meticulous the PO was. If no service records are available just factor in the cost of ALL maintenance into your offer, also consider the more expensive components that can fail as a result. Lucky for us Japanese vehicles are more tolerant of this than Euro ones.
I'm glad you brought this up, I'm going to do some flushing this afternoon.