The subject of this article, brake fluid, is not as sorely neglected as differential gear oil or power steering fluid. Brakes will eventually command the attention of even the most inattentive owner. But we should be paying attention to our brake system long before it screams at us.
Mercedes is one of the few car manufacturers to prescribe a service interval for brake fluid — two years. And there are very good reasons for this. First, brake fluid is highly hygroscopic, readily absorbing water vapor from the atmosphere. If you leave a container of brake fluid open overnight, it will be fatally compromised by the next day. Water in the fluid can boil and vaporize when the brakes are applied, preventing the transmission of pressure from the master cylinder to the caliper. The seriousness of that safety hazard need not be stated. Water also causes corrosion within the brake system. Finally, the fluid within the caliper is subjected to very high temperatures and degrades over time. The fluid in the reservoir might look alright, but there is no recirculation of fluid in this system to provide a visual cue to intervene.
If you live in a humid climate, Mercedes’ prescription may actually not be strong enough. It may be necessary to change the fluid every year. And it is best to do this in the driest part of the year. For most people, the job is best done in the spring, but in swampy Central Florida, for example, the best time to work on brakes is the winter, which is the dry season. Regardless, regular work on the brakes provides more opportunities to inspect the condition of the pads, rotors, and flexible brake lines. The opening and closing of the bleeder screws prevents them from becoming frozen in place by rust. And your caliper seals with love you for it.
While some still swear by the traditional, two-man brake-bleeding procedure, power bleeders are wonderfully effective and make brake bleeding a simple, one-man operation. Power bleeders eliminate the need to constantly monitor the fluid level in the reservoir, thereby reducing the risk of air entering the system and requiring us to start all over again. And they have been demonstrated to reduce the risk of damage to the master cylinder during traditional bleeding.
The standard advice to start with the caliper furthest from the master cylinder holds true. In fact, that right rear caliper seems to be by far the hardest to bleed completely. It is imperative to keep going until the fluid leaving the caliper is completely clean and free of bubbles. A small amount of Teflon tape wrapped around the threads of the bleeder screw can safeguard against any tendency for false air to be sucked in at that point. While such air does not remain in the caliper to cause operational problems, it does make it difficult to tell when the brakes have been bled successfully.
DOT 4 fluid should be used in Mercedes brake systems. If all you can get is DOT 3, that is not the end of the world: it will not harm the system in any way; it simply has a lower boiling point. Whatever you do, do not use DOT 5 brake fluid; this is designed solely for racing applications and does not belong in road vehicles.