The pics above are of a Brake Master cylinder and a Clutch master and slave set. I've been a mechanic for 34 years and worked mainly down south in the cooler places. Cairns humidity is shocking and that's why i do a lot of clutch and brake work. Reason being is brake fluid absorbs water, the high humidity up here makes the life of your brake fluid less, rust occurs inside the cylinder and the cylinder leaks. Expensive.
However the recommended time frame for this is every two years regardless of K.M. But if you change your brake and clutch fluid every 18 months you shouldn't have rust in your cylinders. I use a bendex ceramic dot4 fluid, it's about $18 per 500ml. Yes you can get brake fluid from $12, dot4 but it's probably not sufficent to run here in the tropics. So next time you service, bear in mind your brake and clutch fluid. Something so important but often overlooked.