Brake fluid flush and replace is recommended every 2 years or 20,000 miles, whichever occurs first. I decided to go with whichever occurred last, and did my brake fluid change last week at 4 years and 20,000 miles.

Brake Fluid

Mercedes specifies DOT4+ brake fluid. DOT4+ is MB’s designation for an enhanced DOT4 fluid that has a higher boiling point and lower viscosity at -40F. While many of MB’s fluid specification sheets list approved suppliers and blends (engine oil, for example), the specification sheet for brake fluid, MB331.0, is a material safety data sheet. In section 9, it obliquely references a variety of industry standards, such as FMVSS 116, so that others can spend hours comparing these compliances trying to identify compatible brake fluids.

I started with Pentosin Super DOT4, whose spec sheet cites MB approval reference numbers and a variety of industry standards. I found a local supplier and went to pick up a couple liters, but when the metal containers were placed on the counter, I observed that they were covered with dust, had oxidation pitting and carried no expiration dates. I found another local supplier that stocked LiquiMoly SL6 DOT4, whose spec sheet listed the same industry specs, and specifically a boiling point of >500F and -40F viscosity of <700 mm2/sec, which are the DOT4+ enhancements. The LiquiMoly bottles were plastic, and were labeled with both a date of production (9 months ago) and an expiration date (15 months hence). Brake fluid should be labeled with a 2-year shelf life.

Special Tools

The old “pump and dump” two-person brake fluid servicing technique has been obsoleted by specialty tools designed for brake system servicing. One method uses a pistol type vacuum pump that draws brake fluid out at each bleed nipple, while fluid is added to the brake reservoir. The other system uses a pressurized bottle that forces new fluid into the reservoir, through the brake lines and out each bleed nipple. MB WIS document AP42.00-P-4280GA describes the pressurized bottle method. I purchased a Motive 0109 Black Label European Power Bleeder (about $100) and two collection bottles to perform the servicing. By the way, the Motive bleeder worked flawlessly – no pressure leaks and easy to use.

Procedure

There are several YouTube videos in addition to the Motive instructions and the general guidelines provided in the MB WIS document that are excellent tutorials on using this system to perform the brake fluid replacement. Pulling each wheel one at a time to access each brake caliper’s bleed nipple is the toughest part of the job. But this provided me the chance to acquire accurate measurements on the brake pads and tire treads. About 2 hours was required to complete the service, not including clean-up.

(By the way, my brake pads are still at 10mm after 20K miles – 12mm is the thickness when new.)

Clean-up

Unfortunately, most retailers that accept engine oil for recycling do not accept brake fluid. So I used the kitty litter disposal method. Once the litter has dried out, it can be bagged and disposed of along with other trash. The Motive pressure and collection bottles can be rinsed with isopropyl alcohol, allowed to dry and then stored.



Final Analysis

The extracted brake fluid was indistinguishable from the new fluid, both being transparent amber in color. This made bleeding the old fluid out somewhat guesswork. Since the capacity of the system is spec’d at about ¾ liter, I drew about 1/3 liter from each caliper. The extracted fluid was not discolored, which is good because this indicates there is no unusual oxidation that’s occurred. However, water by itself does not change the brake fluid color. Determining the water content, if there is any, is not so easy. I purchased an inexpensive $25 digital brake fluid tester, but it indicated the old fluid and new fluid were 0.5% and 0.7% water respectively. I took a small amount of the extracted fluid in a glass jar and began adding water with a medicine dropper, but 30 drops of water seemed excessive for the instrument to finally register 2%. There are more expensive testers, and the most reliable is supposed to be a boiling point tester. But even these have to be set for the manufacturer and type of fluid, because every formulation is apparently different.

My brake fluid probably did not require replacing, and if not for MB’s 2 year / 20,000 mile service recommendation, I likely would not have done it. I hadn’t noticed any unusual braking characteristics, and I don’t drive and brake aggressively, and so maybe my fluid still had another 2 years of service in it. In any event, it’s now done and noted in my service records.

I’d be interested in hearing the experiences of other forum members who are also DIY’ers on this topic.

 

Excellent share here with some great information that is valuable to us all. I appreciate your details, findings, and information regarding the products, and process. Great work!

 

If you look carefully I have found the new fluid is slightly lighter in color compared to the old fluid. I don’t have a tester I backlight the motive flush tubing with an LED light... when I see the lighter new fluid come through I let it run a little bit then close off the bleeder valve.

 

Thanks for the suggestion! I'll try this next time. The fluid passing through the somewhat translucent tube was not distinguishable to my eyes, but perhaps the LED backlighting will help. I have some amber colored LEDs -- I'll see if this makes the color difference more pronounced.

 

If your fluid isn't darker, it can be hard to tell, so what I like to do is move an entire reservoir worth through each caliper to be extra sure and give it a good swish. A full flush should use about .5L

 

When I was researching the two alternate methods -- pressure bottle versus vacuum pistol -- the vacuum pistol seemed more appealing. As you say, fill the reservoir and draw it down at each caliper. Then refill and do the next caliper. I decided on the pressure bottle system because some users of the vacuum method report that on some vehicles, the vacuum method tends to suck air into the line through the bleed nipple threads, making it impossible to determine if there is air in the brake line. The pressure bottle method is less efficient for one time use because it takes at least 1.5 liters to fill the bottle and the hose that connects to the reservoir. Except for needing to waste a second liter of brake fluid, the pressure bottle method works well.

 

Here's a picture of the old brake fluid (left) and a sample of the LiquiMoly SL6 DOT4 fluid (right) that I installed. To the naked eye, not any appreciable color difference. As stated in my OP, color doesn't reveal water contamination unless the water has already attacked brake system components and has caused the fluid to darken.

 

Always flush brake fluid every 2 years, regardless of mileage. Brake fluid absorbs moisture. Moisture will degrade braking performance and will cause the caliper pistons to rust. Rust will lead to a seized piston and brake failure.
I had this happen on a Toyota FJ that was 4 years old.

 

This service is one that dealerships charge a fee I am willing to pay. Last year I had the dealership do my flush, I believe it was 150.00 which is well worth it to me.

I would much rather pay the dealership to do a brake fluid flush vs an oil change.

 

For me, it was much more a matter of convenience and having the job done right. I'm a day's drive from the closest MB dealership, and the last time I made a service appointment at one of them, I arrived to learn my appointment was not to have the service completed, but it was only an appointment to drop off my car so that the service department could get around to servicing it sometime the following week. I looked at nearby indy shops -- even Grease Monkey does brake fluid service -- but the most shops work with DOT3 and DOT 4 fluids, and not the MB spec'd DOT4+. I did not want to have my DOT4+ fluid diluted with whatever residual fluid was in their equipment, so I did the job myself. But I agree that $150 for doing the brake fluid service is certainly worth it.

 

I can understand if the vehicle is always parked outside and have more chance of absorbing moisture. What if the the vehicle is always parked in a garage?

 

Brake fluid is hygroscopic and readily mixes with water. That said, brake systems are sealed hydraulic systems that can withstand internal pressures of typically 2000 psi (about 136 times atmospheric pressure) in a hard stop. So the question is if brake fluid doesn't leak out under this kind of internal pressure, how does moisture enter this sealed system when the atmospheric pressure is only 14.7 psi? It's hard to find a definitive answer to this, although it could be that the caliper piston seals are designed asymmetrically, and are much better at holding in pressure than holding out pressure. Thermal expansion and perhaps immersion up to the axles when the calipers are retracting could draw in moisture. Still, with a capacity of perhaps 1 liter of brake fluid and the recommendation that 3% water demands a fluid flush and replacement, this much water -- 30 ml / 1/8 cup / 1 oz. is not a insignificant amount. Brake fluid somehow does absorb moisture and needs to be replaced periodically.

There are a variety of methods for testing the moisture content of brake fluid. The first is appearance -- if the brake fluid is discolored (dark), you're already too late. Conductivity tests (either volt/ohm meters or brake fluid testers) seem to be good at producing inaccurate and inconsistent results, rendering them worthless. There are boiling point and optical refractometers that are likely to produce meaningful moisture content measurement, but cost several hundred dollars and make sense only for a brake service technician. There is also a test strip measurement system (brakebleeder dot com) that measures the amount of copper from the internal brake line plating that has been released into the fluid as a consequence of moisture corrosion. Out of curiosity, I purchased some of these, and my brake fluid measured 10 ppm (replacement is recommended at 200 ppm). But this test strip is assessing the amount of copper in the fluid, not the amount of water, so this indirect measurement certainly can be questioned.

The conclusion seems to be that simply flushing and replacing the brake fluid periodically is the best practice, especially given the cost (about $125) of doing this every 2 years. Like any other maintenance item, vehicle use needs to be considered. If I drove off road and forded axle depth water, or if I lived in a high humidity / high rainfall climate, I would stick with the 2 year or even more frequent flush/replace interval specification. The cost of periodic maintenance is miniscule compared to the cost of replacing calipers and other brake system components.

 

I think my dealer charges more than $125 for the brake service. I do try to get it done about every 2 years, sometimes stretch it a few months beyond that with the "parked inside" excuse.