Not sure if annually was posted but I said every couple of years. The glycol based brake fluid is hydroscopic, i.e. will absorb moisture from the air. This will cause rust to form on the steel parts that it contacts. Often seen in the bottom of the wheel cylinders where it settles often.
I think reading back that Ken might be referring to the trucks that he runs in his business, which are probably driven many miles each year. Most of the older systems are vented and every time you apply the brake there is air sucked into the master cylinder to displace the fluid that has moved into the system. The more times you apply the brake the more times the fluid reservoir is exposed to fresh moisture laden air.
Denny Graham
Sandwich, IL
