Every time the lid or cover on the MC is removed to check the fluid level, air enters the system from the outside allowing moisture to enter the system, and additionally from venting. As the temperature drops, water condenses and immediately mixes with glycol-based brake fluid. Over a period of time, the glycol-based fluid will accumulate water, which can cause corrosion. Additionally, as the water content increases in glycol-based fluids, the flash point is lowered, which can cause the fluid to boil at a lower temperature. This can be a problem for vehicles that experience heavy braking and where elevated brake temperature develop. I agree that DOT 5 should not be used with ABS systems, but for non-ABS vehicles DOT 5 is much better. I don't know what the correct frequency for replacement of DOT 3, 4 or 5.1 fluid should be, but I believe it should b e replaced periodically.