Agree with everyone else on this. Dot 4 changed regularly.
As far as rusting is concerned, I don’t think that the hygroscopic fluid is totally to blame. When you go through a puddle, water is easily able to enter the brake drum. Many people who have suffered corroded cylinders seem to have it most commonly on the nearside (where puddles are most prevalent on the roadside. )
Also, when parked up, moisture can condense inside the drum as it cools. If not driven for a while, I sometimes hear a rear brake squeak going down my driveway but it soon disappears as surface rust rubs off.
For longevity, when fitting new cylinders best to peel back the rubber boot and pack with Ceretec grease or similar, something manufacturers often fail to do.
