2015-2016 Mercedes-Benz C400 Brake Fluid Type and Flush Interval

By Core Brake Parts Tech Team — Circleville, OH warehouse

The 2015-2016 Mercedes-Benz C400 4MATIC (W205) uses a glycol-based brake fluid that carries Mercedes-Benz approval MB 331.0, the DOT 4-class spec named in the C-Class Operator's Manual. Mercedes-Benz USA schedules a brake fluid exchange as part of Service B, which recurs about every 20,000 miles or 2 years, so plan to flush the fluid at least every two years even if the car sits most of the time.

Below is what the spec actually means, why the two-year clock matters more than mileage, how to tell the fluid is due, and what else is worth doing while the system is open.

What Brake Fluid the C400 Takes

The service products section of the 2016 C-Class Operator's Manual, which covers the C 400 4MATIC, says to use only brake fluid approved by Mercedes-Benz in accordance with MB-Approval 331.0. It points owners to the Mercedes-Benz specifications for operating fluids (the BeVo site) for the list of approved products. The manual also warns that fluid which is not approved for Mercedes-Benz vehicles, or not of equivalent quality, could affect the car's operating safety.

MB 331.0 is not an exotic chemistry. It is a glycol-ether fluid in the DOT 4 family with Mercedes' own requirements layered on top, including low viscosity when cold so the ABS and ESP hydraulic unit can move fluid quickly. Brake fluid makers such as ATE explain that low-viscosity fluids exist precisely so ESP and ABS can react quickly, even in extreme cold. When you shop, look for "MB 331.0" on the bottle or confirm the product appears on Mercedes' approved list. A bottle that only says "DOT 4" meets the federal minimum but is not proof it meets the Mercedes spec.

Those are the legal floors from the federal brake fluid standard, FMVSS 116. Quality DOT 4 fluids are published well above the floor; ATE, for example, lists its SL DOT 4 at a minimum of 260 C dry and 165 C wet. The low-viscosity requirement is the part a bargain fluid is most likely to miss.

What not to use: never put silicone DOT 5 fluid in this car, and never use power steering fluid, mineral oil or anything that is not a glycol DOT fluid. DOT 3 fluid is glycol-based and will mix with DOT 4, but it has lower boiling points and does not meet MB 331.0, so treat it as an emergency top-off at most and flush the system afterward.

Flush Interval: Why Two Years Matters

Mercedes-Benz USA lists brake fluid exchange as part of Service B. Service A comes first, at about 10,000 miles or one year, and does not include the fluid change. Service B follows about 20,000 miles or one year after the previous service, then repeats roughly every 20,000 miles or 2 years. Mercedes dealers describe the same thing: Service B adds a brake fluid change and a cabin filter to the Service A items.

The reason is time, not mileage. The C-Class manual explains that brake fluid constantly absorbs moisture from the air, which lowers its boiling point. If the boiling point drops far enough, vapor pockets can form when the brakes are applied hard, and braking efficiency drops. A C400 that covers only a few thousand miles a year still pulls moisture in through the reservoir vent and the rubber hoses, so a low-mileage car needs the flush on the calendar schedule just the same.

That is also why the federal standard publishes a "wet" boiling point. The wet figure is measured after the fluid has taken on moisture, and for DOT 4 the minimum drops from 230 C dry to 155 C wet. The two-year interval keeps the fluid close to the dry end of that range.

The C400 is a heavy all-wheel-drive sedan with a twin-turbo V6. Fast driving, long mountain descents and repeated hard stops all heat the front calipers, which is exactly when moisture-laden fluid causes trouble. If you drive the car hard or take it to track days, shorten the interval rather than stretch it.

Signs the Fluid Is Due or Something Is Wrong

• It has been two years. If you cannot find a brake fluid exchange in the maintenance booklet or service history, assume it is due.
• Dark fluid in the reservoir. The federal standard requires new DOT 3, 4 and 5.1 fluid to be colorless to amber. Fluid that has turned brown is old, though color alone does not measure moisture. A shop can test water content or boiling point directly.
• A pedal that goes soft after hard use. A pedal that feels normal around town but gets long after repeated hard stops is the classic sign of fluid boiling in the calipers.
• "Check Brake Fluid Level" message. The manual says this means there is not enough fluid in the reservoir, the red brake warning lamp comes on and a tone sounds. Mercedes' instruction is to pull over safely, not continue driving, and have the car checked. It also says not to simply add fluid, because that does not correct the malfunction.

Low fluid on this car has two usual causes, and the manual names both: brake pad wear or leaking brake lines. As the front pads wear, the caliper pistons move outward and more fluid sits in the calipers, so the reservoir level falls. Topping it up hides that. If the level is low, check pad thickness first, then look for wet spots at the calipers, hoses and lines.

How the Flush Is Done on a W205

The C400 has ABS, ESP and an electric parking brake, and the manual recommends having brake fluid replaced at a qualified specialist workshop, with the replacement recorded in the maintenance booklet. A shop will normally pressure-bleed from the reservoir so fresh fluid is pushed through each wheel in turn without introducing air into the ABS and ESP hydraulic unit.

A capable DIY owner can do a standard pressure bleed with a reservoir adapter, working from the wheel farthest from the master cylinder to the nearest and never letting the reservoir run low. Keep these points in mind:

• Use a fresh, sealed bottle of MB 331.0 fluid. An opened bottle that has sat on a shelf has already been absorbing moisture.
• Do not let the reservoir drop below the minimum mark during the bleed. Air drawn into the hydraulic unit may need a scan tool routine to purge.
• Keep fluid off paint. Glycol brake fluid attacks paintwork, so cover the fender and rinse any spills with water right away.
• Collect the old fluid in a closed container and take it to a recycling or hazardous waste drop-off.
• After the bleed, confirm a firm pedal with the engine running before moving the car, then test at low speed.

Difficulty is moderate. The job itself is simple, but the electric parking brake and the ABS/ESP unit leave less room for mistakes than an older car. If the pedal is not firm after a normal bleed, stop and have a shop finish it with a scan tool.

What to Replace at the Same Time

A fluid flush is the natural time to look at everything the fluid touches. Pull the wheels and check pad thickness, rotor condition and the flexible hoses for cracks or swelling.

• Front pads. If the fronts are close to the wear sensor, replace them now so you only bleed once. For a C400 driven hard, the EBC Yellowstuff front brake pads for the 2015-2016 C400 4MATIC are a high-friction street and track compound. For daily driving, the EBC Redstuff ceramic front pad set for the C400 is the quieter, lower-dust choice. Browse more options in our brake pads collection.
• Rotors. The manual notes that Mercedes approves specific brake discs and pads for these cars, and that lightweight discs should be protected from shock loads when wheels are changed. Replace rotors that are below minimum thickness or deeply grooved. See our brake rotors.
• Hoses. Any rubber hose that is cracked, swollen or seeping should be replaced while the system is being flushed anyway. Browse brake hoses.
• Caliper slides and hardware. Clean and lubricate the slide pins and replace worn clips. Our brake hardware kits cover the small parts.

Frequently Asked Questions

What brake fluid does a 2015-2016 Mercedes C400 use?

Brake fluid approved to Mercedes-Benz MB 331.0, per the C-Class Operator's Manual. It is a low-viscosity DOT 4-class glycol fluid. Check the bottle for the MB 331.0 approval rather than relying on a plain DOT 4 label.

How often should I flush the brake fluid on my C400?

Mercedes-Benz USA includes a brake fluid exchange in Service B, which recurs about every 20,000 miles or 2 years. Because moisture absorption depends on time, treat the two-year mark as the deadline even on a low-mileage car.

Can I use regular DOT 4 or DOT 3 fluid?

A fluid that only meets the federal DOT 4 minimum may not meet Mercedes' low-viscosity and quality requirements. DOT 3 has lower boiling points and does not meet the spec. Use an MB 331.0-approved fluid, and if you had to top off with something else, flush the system soon.

My car says Check Brake Fluid Level. Can I just add fluid?

Mercedes says no. The warning means the level is low, which usually points to worn pads or a leak. Pull over safely, do not keep driving, and have the brakes inspected. Adding fluid does not fix the cause.

Can I mix brands of brake fluid?

Glycol-based DOT 3, DOT 4 and DOT 5.1 fluids will mix, but the blend performs no better than its weakest part. Never add silicone DOT 5 to this system. The best practice is a full flush with one MB 331.0-approved fluid.

Sources

1. Mercedes-Benz USA, 2016 C-Class Sedan Operator's Manual - MB-Approval 331.0 brake fluid, moisture absorption and boiling point warning, Check Brake Fluid Level message, pad wear and leaking lines as causes of low fluid, lightweight disc note, C 400 4MATIC coverage
2. Mercedes-Benz USA, Service and Maintenance - Service A and Service B intervals, brake fluid exchange included in Service B
3. Mercedes-Benz of Los Angeles, Service B information - Service B at 20,000 miles or 2 years including a brake fluid change
4. 49 CFR 571.116, FMVSS No. 116 Motor Vehicle Brake Fluids - DOT 3, DOT 4 and DOT 5.1 dry and wet boiling point minimums, -40 C viscosity limits, colorless to amber requirement
5. ATE Brake Fluids - low-viscosity fluid lets ESP and ABS react quickly, SL DOT 4 boiling points, follow OEM change interval