Be aware that removing the radiator drain plug can be disastrous. Some from that era detached themselves from the radiator, which required removal and replacement or if out of warranty, repair. Because of this, most, remove the lower radiator hose to drain the system.
That said, after this happened to mine at the dealers, I stopped draining the system altogether. Instead, I evacuate the reservoir once a month and replenish with 32 ounces of "50/50 Prestone Yellow" once a month. To accelerate a full replacement in this manner, one could do it once a week or after every ride until they have for the most part introduced all fresh coolant. It's so much easier this way. And, if you have any doubt about how well it works, NAPA sells coolant test strips that will settle those fears.
