Nothing is more important to safe driving than effective, reliable brakes, so we evaluate them at every service inspection. We check all the common things like brake pad and brake shoe lining thicknesses, but we also continue on where other shops might stop (brake pun intended). We inspect the integrity of rubber flex-lines for signs of wear or damage, we verify parking brake function and adjustment, we inspect brake discs and drums for wear, and we measure water content within the brake fluid.
Brake fluid is a highly critical element of the braking system and has a special set of requirements: it has to have low viscosity, even at extremely low temperatures, so that it flows freely through the small lines, hoses, and anti-lock brake control components. It has to be compatible with rubber and plastic materials in flex-lines. And most importantly, it has to have a very, very high boiling point. If brake fluid were to overheat and boil, you’d immediately lose all brake functionality! Modern brake fluid is capable of filling all of these requirements, but it has one major drawback– it’s hygroscopic, which means that it absorbs moisture from the surrounding environment. Water molecules leach through the brake fluid reservoir, reservoir cap, brake flex-lines, rubber caliper seals, etc., contaminating the brake fluid. Water contamination results in a dramatically lower boiling-point, which can be catastrophic for braking safety and reliability. But water in the brake fluid will also cause long-term corrosion damage to every metal part of the hydraulic system, including the master cylinder, metal hydraulic lines, calipers, wheel cylinders, and the anti-lock brake hydraulic control unit.
Because of this range of factors, we measure water content with every service inspection. Anything less than two percent water is considered fine, and not in need of replacement. Between two and three percent water content is considered borderline, and indicates that brake fluid should be replaced at that point, or in the near future. Three percent or greater indicates that brake fluid replacement is due immediately, for both the immediate safety of the brakes, as well as the long-term durability of the individual brake components.
There’s a few different methods for replacing brake fluid. The most common method we utilize is to raise the vehicle about a foot off of the ground, remove the wheels, remove the fluid reservoir cap, draw the old fluid out of the reservoir, refill with fresh fluid. We then draw new fluid through the lines to the wheel cylinders and calipers using a vacuum device, refreshing the brake fluid for each of the hydraulic circuits in the system until a total of a liter of new brake fluid has flushed out the old fluid. Then its back on with the wheels, the correct fluid level is confirmed, the fluid reservoir resealed, and a verification of brake system functionality is performed to round out a proper brake fluid flush.
When it comes to disc brake pads, we check them with every service inspection, too. Three millimeters is considered the minimum acceptable thickness for brake pads, which holds true for both the German and Japanese cars that we service. Where those two groups of cars differ is in the material used in the brake disc (also interchangeable with the term ‘brake rotor’). Japanese cars tend to use a very hard brake disc material, which doesn’t wear out easily, but is more prone to warping over time. German cars use softer brake disc material which is highly resistant to warping and tends to give better vehicle stability while braking at highway speeds, but the brake disc material wears much faster. What this means is that Japanese cars may be able to reuse brake discs with each new set of brake pads. German cars, however, almost always require brake disc replacement with each brake pad set.