The subject of this article, the brake fluid, is not as seriously neglected as differential oil or power steering fluid. The brakes will eventually catch the attention of even the most inattentive owner. But we should pay attention to our braking system long before it screams at us.
Mercedes is one of the few automakers to prescribe a service interval for brake fluid – two years. And there are very good reasons for that. First of all, the brake fluid is highly hygroscopic, easily absorbing the water vapor from the atmosphere. If you leave a reservoir of brake fluid open during the night, it will inevitably be compromised the next day. The 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 severity of this security risk does not need to be indicated. The water also causes corrosion in the braking system. Finally, the fluid inside the caliper is subjected to very high temperatures and degrades over time. The liquid in the tank may look good, but there is no recirculation of liquid in this system to provide a visual indication for intervention.
If you live in a humid climate, Mercedes' prescription may actually not be strong enough. It may be necessary to change the fluid each year . And it is best to do it in the driest part of the year. For most people, work is best done in the spring, but in the central Florida swamp, for example, the best time to work on the brakes is the winter, which is the dry season. Whatever the case may be, regular work on the brakes offers more opportunities to inspect the condition of brake pads, rotors and flexible brake lines. Opening and closing the bleed screws prevents them from being frozen by rust. And the stirrup seals with love for you.
While some swear by the traditional brake bleeding procedure, power bleeds are wonderfully effective and make bleeding the brakes a simple one-man operation. Power traps eliminate the need to constantly monitor the level of fluid in the tank, reducing the risk of air entering the system and forcing us to start all over again. And it has been shown that they 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 is true. In fact, this right rear caliper seems to be by far the most difficult to bleed completely. It is imperative to continue until the liquid coming out of the stirrup is completely clean and free of bubbles . A small amount of Teflon tape wrapped around the threads of the bleed screw helps prevent any tendency to suction false air at this location. Although such air does not remain in the caliper to cause operational problems, it is unclear when the brakes have been purged successfully.
DOT 4 fluid should be used in Mercedes brake systems. If all you can get is DOT 3, it's not the end of the world: it will not hurt the system; it simply has a lower boiling point. Whatever you do, do not use DOT 5 brake fluid; this is designed only for racing applications and does not belong to road vehicles.