2017-2023 Mercedes-Benz E350 Brake Fluid Type and Flush Interval

By Core Brake Parts Tech Team — Circleville, OH warehouse

The 2017-2023 Mercedes-Benz E350 (the W213-generation E-Class) calls for brake fluid approved to Mercedes-Benz specification sheet 331.0, sold as DOT 4 Plus. That is a low-viscosity DOT 4 that also meets ISO 4925 Class 6. Mercedes schedules a brake fluid exchange with Service B, which works out to every 2 years or 20,000 miles, whichever comes first.

Below is what that spec actually means, why ordinary DOT 3 or bargain DOT 4 is the wrong call on this car, how to tell your fluid is overdue, and what to plan for when you do the flush.

What Brake Fluid Does the E350 Use?

Mercedes does not just say "DOT 4" in its service literature. It approves fluids against its own operating-fluids sheet, 331.0, and the genuine Mercedes-Benz fluid carries that approval on the label along with the DOT 4 rating. Genuine Mercedes DOT 4 Plus is listed as meeting MB 331.0, ISO 4925 Class 6, SAE J1704 and FMVSS 116 at the same time.

The important part of that list is ISO 4925 Class 6. Class 6 is the low-viscosity category. The federal DOT 4 standard only caps cold viscosity at a level that is fine for a plain hydraulic system, but the E350's ABS and stability-control hydraulic unit has to pulse fluid through small valves very quickly, including on a cold morning. Thin fluid moves through those valves faster, which is why Mercedes and other European makers moved to Class 6 fluids for cars with electronic stability programs.

The table shows the gap clearly. A fluid that only just meets the federal DOT 4 minimum is legal, but it can be more than twice as thick in the cold as a Class 6 fluid. Continental's ATE data sheet for its SL.6 DOT 4 describes its cold viscosity as roughly half that of many other DOT 4 fluids on the market, and that is exactly the property MB 331.0 is written around.

Can You Use Plain DOT 4 or DOT 3 Instead?

Do not use DOT 3. It has a lower boiling point floor than DOT 4, and it is not what the car was engineered for.

Plain DOT 4 is chemically compatible, since glycol-based DOT 4 and DOT 5.1 fluids can be mixed without damaging seals. ATE states its SL.6 can be mixed with other fluids meeting ISO 4925 Class 6, DOT 5.1 or DOT 4. The catch is that ATE also says optimal stability-control function at very low temperatures is best achieved with Class 6 fluid. On an E350 the practical rule is simple:

• For a full flush: use genuine Mercedes DOT 4 Plus or another fluid that lists MB 331.0 approval, or at minimum a fluid labeled ISO 4925 Class 6.
• For a small top-off in a pinch: a sealed bottle of a quality DOT 4 will not hurt the system, but plan to flush back to the correct fluid at the next service.
• Never: silicone DOT 5. It is not compatible with glycol fluids and does not belong in this car.

Always pour from a freshly opened, sealed container. Brake fluid starts pulling moisture out of the air the moment a bottle is opened, so a half-used bottle that has sat on a garage shelf is already partly spent.

Flush Interval: Every 2 Years or 20,000 Miles

Mercedes-Benz builds the brake fluid exchange into its Service B visit. Mercedes dealers describe the result the same way: following the Service A / Service B schedule as intended means a brake fluid replacement every 2 years or 20,000 miles. The time limit matters most for E350 owners who drive relatively few miles. A car that covers only a few thousand miles a year still needs fresh fluid at the two-year mark, because the fluid is degrading whether the car moves or not.

Why so often? Glycol brake fluid is hygroscopic: it absorbs water. Mercedes dealers note that over time the fluid gets dirty and starts to retain water. The FMVSS 116 "wet" boiling point in the table above exists precisely because DOT 3 and DOT 4 fluids lose boiling-point margin as they take on water. Water in the fluid also promotes corrosion inside the calipers, brake lines and the ABS/ESP hydraulic unit, which is by far the most expensive part of the system to replace.

Signs Your E350 Brake Fluid Is Overdue

• Two years or more since the last exchange. Check your service records or the service booklet. If there is no record, assume it is due.
• Dark fluid in the reservoir. Fresh DOT 4 is clear to light amber. Brown or murky fluid has been in there a long time.
• A pedal that goes softer after hard use. Fade on a long downhill or after repeated hard stops can point to fluid that has absorbed water and is boiling at a lower temperature.
• Low reservoir level or a brake fluid warning. The level drops slowly as pads wear, but a warning message or a fast drop means you should look for a leak before you just add fluid.

A spongy pedal can also come from air in the lines, a failing caliper seal or a master cylinder problem, so a soft pedal is a reason to inspect, not just a reason to flush.

How the Flush Is Done and What to Replace at the Same Time

A fluid exchange pushes all the old fluid out of the reservoir, lines and calipers and replaces it with new fluid, generally working from the corner farthest from the master cylinder toward the closest. On a car with a modern ABS/ESP unit, shops typically use a pressure bleeder so fluid is pushed through without letting the reservoir run dry and pulling air into the hydraulic unit. If you do it at home, never let the reservoir drop below the minimum mark while bleeding, and follow the factory service information for the bleed sequence and any scan-tool routine the hydraulic unit may require.

A flush is also the right moment to look over the rest of the system:

• Pads and rotors: the wheels are often off anyway. If pads are near the limit, do them now. We carry Power Stop rear ceramic brake pads for the 2017-2023 E350, plus our full range of brake pads and brake rotors.
• Bleeder screws and caliper condition: a seized bleeder or a sticking caliper piston shows up during a flush. See our brake calipers if one is damaged.
• Rubber brake hoses: look for cracking or bulging while you are at each corner. Replacement brake hoses are best installed during a flush, since the system gets bled anyway.

How Hard Is the Job?

For a shop, a brake fluid exchange is routine work. For a home mechanic it is a moderate job: the mechanical part is simple, but getting every last bubble out of a system with an electronic hydraulic unit is where DIY flushes go wrong. If you have a pressure bleeder, the correct MB 331.0 fluid and access to the factory procedure, it is very doable. If the pedal feels any different after you finish, have the system checked before driving.

Plan on enough fluid to push fresh fluid clearly through every bleeder, not just to fill the reservoir. Buying in sealed containers sized so you open only what you need keeps the leftovers fresh. ATE's data sheet, for example, rates its fluid for storage of up to 5 years only in the original closed container.

Frequently Asked Questions

What brake fluid does a 2017-2023 Mercedes-Benz E350 take?

Brake fluid approved to Mercedes-Benz sheet 331.0, sold as DOT 4 Plus. It is a low-viscosity DOT 4 that also meets ISO 4925 Class 6, SAE J1704 and FMVSS 116.

How often should I flush the brake fluid on my E350?

Mercedes includes the brake fluid exchange in Service B, which means every 2 years or 20,000 miles, whichever comes first. Low-mileage cars still need it at the two-year mark.

Is DOT 4 Plus the same as DOT 4?

DOT 4 Plus meets the DOT 4 standard and more. The key difference is much lower viscosity in the cold (ISO 4925 Class 6), which helps the ABS and stability-control unit modulate pressure quickly. Plain DOT 4 only has to meet a much higher cold-viscosity limit.

Can I mix DOT 4 Plus with regular DOT 4?

Yes, DOT 4 and Class 6 fluids are compatible, so a small top-off will not cause damage. For a flush, use MB 331.0 or Class 6 fluid so you get the full low-viscosity benefit. Never mix in silicone DOT 5.

Why does brake fluid need changing if it never gets used up?

It absorbs water from the air over time. Water lowers the fluid's boiling point, which can cause a soft pedal under hard braking, and it promotes corrosion in calipers, lines and the ABS/ESP hydraulic unit.

Sources

1. Fletcher Jones Mercedes-Benz - Brake Fluid Replacement - brake fluid exchange is part of Service B, every 2 years or 20,000 miles; fluid retains water over time
2. Mercedes-Benz of Tyler - E-Class Service Schedule - E-Class brake fluid exchange at 20,000 miles; moisture contamination
3. 49 CFR 571.116 (FMVSS 116) - Motor vehicle brake fluids - DOT 3 and DOT 4 dry and wet boiling point minimums and maximum viscosity at -40 C
4. ATE Technical Data Sheet TD 007 - DOT 4 SL.6 - Class 6 boiling points and 700 mm2/s cold viscosity, suitability for ESP systems, mixing compatibility, storage life
5. Mercedes-Benz Genuine Brake Fluid DOT 4 Plus product data - genuine fluid meets MB 331.0, ISO 4925 Class 6, SAE J1704 and FMVSS 116
6. Genuine Mercedes-Benz Brake Fluid MB331.0 DOT4 - genuine fluid labeled DOT 4 with MB 331.0 approval