While Japanese bicycles still do use calliper brakes for the front wheel, most are equipped with disc brakes for the rear, even a regular ママチャリ (mama chari), which is a bicycle homemakers use to run errands with. It is an abbreviation of the term mama chariot.
When I had my bicycle caught in the rain back when I was still in Aichi, my disc brakes started squeaking each time I press on the brake handles. It was a little annoying for me for the very simple reason that it was noisy and that it was fairly new when it happened. So when I moved to Tokyo, having no way to bring my bicycle along, I decided to sell it and buy a new one after I arrive.
When I finally got a new bicycle, I took care not to let it get caught in the rain. Even though the apartment I stay in doesn’t provide a sheltered bicycle parking area, I figured that if I parked it nearer to the building, the brake wouldn’t get wet enough to turn squeaky. It went well for a couple of months until I left it in the open area one day on the night it rained. It then turned squeaky and I was resigned.
Some time later, however, I found a new life to the squeak. And this would be why exactly you should leave your bicycle in the rain. While bicycles are definitely equipped with a bell, it is usually impolite to ring it, what more in a society like Japan. I have never sounded the bell at anyone to give way but I’ve mostly still managed to get people noticing I’m coming from behind, thanks to the squeaky brakes. With it, I no longer have to cycle quietly behind people hoping they would notice my presence and open up a tiny path for me to pass. All I have to do is to hit the brakes for the squeak. The best part is, to those people, I was being nice pulling on the brakes to slow down.
The conclusion is, stop taking good care of your bicycle. Just leave it in the rain. It’ll serve you well.