Why does BMW “recommend” a complete brake flush every 2 years, other than to make more money? OK, I’m obviously cynical, but I’ve owned many vehicles for 6+ years and NEVER replaced the brake fluid without any deterioration of braking. I’ve read that moisture can contaminate the brake fluid- hence the brake flush, but what if I don’t bother? Am I going to hurt anything?

Brake fluid absorbes moisture. That moisture tends to collect in the brake calipers where it does several bad things.

First, it causes corrosion in the caliper piston and bore. This can lead to sticking or seized caliper pistons resulting in the need to replace them with new or rebuilt units. Contrary to popular beleif, calipers are not wear items and should last the life of your car if you bleed your brakes regularly.

Second, the moisture can become a safety issue. "Wet" brake fluid boils at a much lower temperature than "dry" brake fluid. If you are going down a long hill and manage to heat up your brakes to the point that the water in the calipers starts to boil, it will form gas bubbles. These gas bubbles are compressible and will result in something known in racing circles as "fluid fade." (There are a number of ways to make your brakes fade and boiling brake fluid is just one of them.) The bottom line is that your brake pedal may become spongy or may sink all the way to the floor before the gas bubbles are compressed enough to allow your brakes to work. The only way to get these gas bubbles out is to bleed the brakes and replace the old fluid with fresh fluid.

If you check out my brake fluid web page, you'll see that there is a huge difference between the dry and wet boiling points. You'll also see that the BMW-recommended ATE TYP 200 fluid is better than most stuff you can buy at the auto parts store.

Bleeding your brakes is easy and there's no reason that you should overpay someone else to do a careless job of it. I put together a brake bleeding web page for the Impala SS folks, but the principles are the same for our E39s.

Dave Z.