SBC Brake Flush Revisited (Again) - In Great Detail - Mercedes-Benz Forum
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post #1 of 7 (permalink) Old 11-27-2007, 12:56 PM Thread Starter
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Lightbulb SBC Brake Flush Revisited (Again) - In Great Detail

The main purpose of this post is to create detailed discussion and analysis of the DIY flushing of a SBC brake system. I am hoping others will read this lengthy post and have deeper knowledge and understanding of the SBC circuit and help to set me straight. I warn you now, this is a LONG detailed analysis.

Being a new 2006 E320 CDI owner and avid DIY maintenance guy, I am diving into my first Benz maintenance with the brakes, as the fluid is a nice carmel color now. I too have the SBC system and am thankful to all those that have posted so much. I am a hydraulics engineer (25 year veteran) and been involved in many complex braking systems, although my real forte is power transfer. I have digested the schematic that I have found on the SBC system. With that study, I understand why Benz says you MUST use the Star Diags to flush the system. They are correct if you want a true 100% flush, as this gets the SBC pump involved in the flush. To get a TRUE 100% flush and insure you have all water and dirt out, you have to use the SBC pump. HOWEVER, results that are good enough for my satisfaction (and I am quite the perfectionist) can be achieved at home with extra time and brake fluid. But we should all be made aware of a weakness in the DIY pressure bleed procedure. I would like to discuss the brake fluid and the schematic. The URL for the Benz document I reference herin is:
My discussion assumes you have this document in front of you as I refer to it throughout.

First, the brake fluid itself; As discussed elsewhere in this forum, DOT 3, 4 & 4+ brake fluids are a complex mix of various types of phosphate esters and ethylene glycols. They are hydrophilic (water absorbing) and not compatible with typical buna-N and Viton type oil seals. EPR (ethylene propylene) is the seal material of choice. Unfortunately, EPR seals swell and dissolve when exposed to petroleum based oils, so that is why it is imperative you never get even a small amount of oil mixed with brake fluid, else you are looking at a complete system rebuild. Water makes brake fluid's additives "unfriendly" to seals and metal parts. You cannot tell by visual inspection if water is present in the brake fluid. Over time, it does absorb water from the atmosphere. If you want to do a quick test (but quite revealing) to see if your brake fluid has water in it, you can perform an "old school" sizzle test. Take a piece of sheet metal and hold it horizontally with vice grips. Heat it from below with a propane torch to get it well over 300F. Careful not to get it too hot, as brake fluid will burn and smoke noxious gas if gotten too hot. Using a glass eye dropper, pull some brake fluid from your master cylinder reservoir and place a drop on the hot sheet metal. If you have water in your brake fluid, it will boil out and "sizzle" on the hot sheet metal. This test is a bit extreme for brake systems however, as you should replace your fluid AT LEAST every other year anyway. If you get to the point where your brake fluid sizzles, you are not doing maintenance often enough or you live in a tropical rain forest.

Referring to the Benz .pdf document, I will explain why the DIY flush is not complete when done at home and the specifics of each of the valves in the schematic, as I have interpreted their functions. Note that I still have a number of questions that need to be answered to fully understand the brake system fluid flow. Here are the components with better explanations of their functions than provided in the .pdf document, ignoring pressure sensors.

3 - Hydraulic accumulator - stores pressurized fluid for immediate brake response from SBC system
y1 & y2 - Manual override valves - for when SBC system fails. Note that you only have front brakes when the SBC system fails.
7 & 8 - Pressure isolators for use with manual override y1 & y2 valves. These pistons keep high pressure fluid from backing up into the SBC system when in manual mode.
y6, y8, y10 & y12 - proportional sequence/reducing pressure control valves. These are the actual valves that make the SBC system work; i.e. - your "brake-by-wire" valves.
y7, y9, y11 & y13 - pressure release valves for relieving pressure from the brake calipers when you take your foot off the brake pedal.
y3 & y4 - Pressure balance valves. These insure that the pressure on the left & right brakes are equal on each axle or isolates them for independent pressures when braking on a curve.
m1 - The SBC pump that provides all of the braking pressure during normal operation.

Important items of note.
1 - For those that want to argue symantics; Pumps provide flow, not pressure. Pressure is caused by a resistance to flow. I use the word pressure interchangeably with flow to simplify the concepts for those non-hydraulically experienced.
2 - When the car in not in operation, all valves are normally open, with the exception of the proportional sequence/reducing valves, y6, y8, y10 & y12, which are closed off.
3 - Presumably, the accumulator (3) has pressure stored between it and the sequence valves when the car is turned off. This pressure is about 2300 PSI.
4 - Return line to master cylinder reservoir comes in the top of the reservoir. Normally, a hydraulic return line returns UNDER the fluid level to insure no air is introduced into the system. I cannot determine by casual observation if the reservoir is actually made in such a way that the return does extend below the fluid level internally. This is important to note if using something akin to a Motive pressure bleeder for flushing brakes. Logic wants me to believe that this return does indeed return under the fluid level, although I cannot make this assumption.
5 - Not shown on the schematic are the bleeder valves on the brake calipers. When I refer to bleeding the brakes herein, the fluid will flow out of this valve from the caliper, although this is not shown in the schematic.
6 - There is no check valve shown on the outlet of the pump before the accumulator. I must assume it actually exists, else all accumulator pressure would flow back through the pump into the master cylinder reservoir when the pump is shut off.

Referring to page 51 of the .pdf document which shows the attachment of equipment for a Benz shop bleed, there are a number of difficulties that I can envision when trying to bleed the brakes using a Motive type bleeder on a SBC system at home. I first must assume that the car is off and nothing is being done to make the SBC system work. The Motive bleeder will have 20 PSI on it or so.

If there is stored pressure on the accumulator and a check valve on the outlet of the pump, it should be noted that the Motive bleeder will have no effect on the portion of the system between the pump outlet and the sequence valves. This part of the system would be held at 2300 PSI and obviously, a 20 PSI supply will not overcome this high pressure. Therefore, the fluid in the accumulator and its associated lines will not be exchanged.

Note that the back brakes on a SBC system are powered only through the SBC system, as there is no manual override. Since the SBC is presumably held at high pressure, the only bleed path for the rear brakes would be through the return lines via valves y11 & y13. If the return line does not return under the fluid level in the reservoir, it will require that air enter the return line until the fluid pushes up to the top of the return chamber and allows clean fluid to enter. So a big air bubble would be pushed through the system and eventually be bled out through one of the rear calipers. If this is true, the right rear caliper should DEFINITELY be flushed out first (standard flush procedure anyway) so that extra air is not introduced into the front brakes' isolation piston assemblies.

The front brakes have two flow paths from the reservoir. One is via the return line and valves y7 & y9; the other is through the manual override valves, y1 & y2. However, the valves y7 & y9 lead to a dead end path on the backside of the isolation pistons. Therefore, when we get clear fluid from bleeding the front brakes, we know that we have bled the system via valves y1 & y2. This is a different path than followed by the flushing of the back brakes.


There seems to be a good number of paths that are not exchanged in the flushing process when done manually in one's home garage. The areas associated with the non-spring side of the front brakes isolation pistons will not have fluid exchange, nor will the entire pressurized portion of the SBC system between the pump and the sequence valves. This means that you will leave a decent percentage of the old fluid in the system when performing a DIY brake flush with a Motive type bleeder at home. Once you start working the brakes in normal operation, this old fluid will mix with the new fluid leaving considerable contamination in your brakes.

At the Benz shop, I can envision that the SBC pump and valves would be sequenced in such a way that 99% of the fluid in the system is exchanged. The SBC system could be sequenced in such a way as to allow for 80% of the system to be bled through the right rear bleed valve. Then one would just bleed out the remaining calipers. I must admit, if you have the SDS system, this is pretty slick. But I am not about to shell out thousands of dollars for the SDS.


Examining line lengths, the storage size of the accumulator and possible non-exchanged fluid in the system, I would estimate that using a DIY method at home would exchange about 3/4 of the total brake fluid in a W211 SBC brake system. If you let your brake fluid get really bad, this is not a good enough exchange. However, if you are flushing your brakes more often than Benz's recommendation using this DIY method, this percentage of exchange should keep your fluid in good condition. Again, this assumes you don't live in a tropical rain forest.


Assuming my SBC system analysis above is correct, I intend to flush my brakes with a Motive bleeder once a year. This should give me a high enough exchange of the fluid to insure I have good fluid in my SBC system and keep the brakes in good working order for years to come. The complete brake flush at Benz which uses the SBC system and the Star Diagnostics costs $200+ after tax. I would like to avoid this expense and inconvenience of yet another "dealer only" piece if maintenance. I hate being held over the barrel.

If someone here has a more thorough understanding of the SBC hydraulic circuitry and can point out something I missed, PLEASE let me know. Let me know your thoughts.

Last edited by smyers; 11-27-2007 at 01:11 PM.
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post #2 of 7 (permalink) Old 11-27-2007, 04:05 PM
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A "short version" document from Mercedes:

General information on brake flushing:
Industry Adopts Brake Fluid Replacement Guidelines, Bill Williams, Brake & Front End, May, 2004

Kent Christensen
'07 GL320CDI, '10 CL550
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post #3 of 7 (permalink) Old 11-27-2007, 04:40 PM
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let me ask you this, how much money are you saving by doing it your self

mb brake fluid= $15 qt x 5 =$75 plus tax
mb flush = 200 plus tax
a mb sbc brake flush is good for 3 years 30,000 miles.
DIY brake flush isnt even good for a year becuase your only getting 1/3 of the fluid out
so realy you need to do it 3 times a year to equal a mb brake flush.
if you flush it once a year for the three years a normal flush is worth it will cost you
$225 plus tax. and its not even taking out all the bad fluid.
plus the hassel and time it takes and the cost of a flush machine. after all this cost you could do three brake flushes. thats 9 years of protection with out doing anything your self. you can sit back a sip a latte for the hour it takes.
have fun with your flushing, all your getting out of it is the experience of messing up your own car. and spending more in the long run


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post #4 of 7 (permalink) Old 11-28-2007, 05:48 AM Thread Starter
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Quantify Detail Please


Thanks for the response. You may be a Benz Tech, but I do want to set a few things straight.

-Please reread the first line of the beginning this post. It says, “The main purpose of this post is to create detailed discussion and analysis of the DIY flushing of a SBC brake system.” This post is meant to get some discussion on the technical aspects of the DIY flush and SBC system, not discuss how I will “mess up” my own car. It’s my car and I will do with it as I will. I even point out in my original post how the dealer flush is quite superior to a DIY flush. I want to discuss the DIY flush from an engineering perspective, not a mechanic’s perspective. This takes the discussion to a completely different level. I do understand that you are a tech and as such, you have every right to defend your work. And Benz techs ARE THE BEST I have ever experienced. That is a given. But let’s keep the discussion on track.

-I enjoy doing my own maintenance and find it rewarding. More people should if they have the skill set, IMHO. By doing so, you become much more familiar with your car’s systems and can get things repaired before they are a real problem, as you find them on your inspections. Is it pride? Perhaps. No matter the reason or the risk of messing up my car, I will still do it. I’ve not messed up a car yet or cost myself money. However, this is again off topic for this post.

-I apologize for pointing this out again, but I am a hydraulics engineer and been in the industry for 25 years. I am no spring chicken, as my grandma would say. I know how to analyze hydraulic systems in ridiculous detail. That said, I will continue.

-Benz DOT 4+ brake fluid from the dealer - $10 per liter, not $15

-Benz does not have a magic wand to make their flush last more than 2 years. It is a matter of moisture (and dirt) ingestion. The brake system breathes at the filler cap and pulls in moisture, which the fluid loves to absorb. To go more than 2 years is asking for trouble, unless you are in very dry area. What you say at 3 year intervals disagrees with every other Benz tech I have spoken with. I am at 18 months now and my fluid is a carmel color.

-I drive 50K per year. So if the Benz shop brake flush costs $225 and is good for 30K, it will cost me about $375 per year. Suddenly, doing my own is more attractive. If I am correct in my volume calculations mentioned below and I were to do a DIY brake flush every 6 months which will take less than 1 hour, I estimate I will have $20 in fluid at most for each flush, not counting labor. The real debate would be on the intermix ratio of old to new fluid and what is sufficient for optimum cost effectiveness VS system life. If I hypothesize that I have 0.5% water and other contaminants in my fluid and I get a 2/3 exchange (the debatable figure), then my contaminant level would be down to .166%. (I can tell you from experience that these figures are too high, but they work for the sake of this discussion. They are more than likely down in the .05% range or less.) If I do a flush at say every 25K (6 months for me) and my ingestion rate is constant (which it will be), then I could estimate that I would ingest another .25% in the next 25K miles, making it a total of .416% contaminants on the next flush. On the next flush, this would put me down to .138% contaminants when I finish the flush and .388% when I start the next flush… and so on. It will level out at about .125% contaminants after a DIY flush over time. If all this is true, then the most cost effective way for me to go about flushing my brakes and yet keep the system in very good condition and considering my mileage is to do a DIY flush every 6 months. Then, after I do three of them, pay for a Benz shop flush at the end of 2 years.

-According to Benz’s own document, it takes 1.5 liters to flush the system at a dealership. Perhaps this is misinformation. Perhaps not. But certainly the only way it takes 5 liters is through un-conservative work.

-Yes, there is the cost of a flushing unit. But I already have one. Even if I didn’t, they only cost $50-$60 bucks and it is a one time purchase.

-On how long it REALLY takes out of my day to go to the dealership for service VS a DIY brake flush; From the time I get it out the flushing unit, clean it, introduce new fluid, suck out the master cylinder reservoir and refill it and then finish the flush, then clean up and put my tools away, I can be done in 30-45 minutes at most. 1 hour if I really drag my heels. I’ve done it on many cars. It takes me 20 minutes just to get to the dealership, at least 1.5 hours there (Assuming they can get me in when they say. Sorry, but 1 hour is fantasy land here.) and 20 minutes to get back home. That’s 1 hour, 50 minutes. If they can’t get me in right away, then it’s the loaner and I have to go back later. So that is at least 20 minutes x 4, plus another 20 minutes for paperwork and chit-chat at both ends of the visit for a total of 1 hour 40 minutes.

-You say I will only get out 1/3 of the brake fluid on a DIY flush. Now if you would like to tell me how many cc’s or cubic inches of fluid I will leave in my system exactly when I do a DIY flush, then we would be discussing the technical aspects. So if you are a Benz tech, how many is it? A DIY flush will get about 2/3 to ¾ of the fluid out the system if you use something akin to a Motive flushing unit, according to the math and system segment volume calculations. Please come at me with more than a blanket statement of “your only getting 1/3 of the fluid out”. Please quantify the statement. I hope you can document this as it doesn’t come close to agreeing with my math on caliper volume, line lengths & volume, reservoir volumes, etc. I would like to see the documentation on this if it can be proven. Again, this post is meant to create detailed & technical discussion, guess and conjecture. There is enough of that on here already. I need more than “what I think”. Quantify, quantify, quantify.

I do appreciate your passion for what you do. Please, by all means, keep up that passion!

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post #5 of 7 (permalink) Old 11-29-2007, 06:07 AM Thread Starter
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Thumbs up Good Info - brake fluid article


Thanks for your informative links, especially the one on brake fluid. This is the kind of information that is useful. I found the article interesting. The type of testing described in the article come from standards set by the hydraulics industry. The issue of brake fluid life (and hydraulic fluid life in general) is indeed a complex one.

It has been known in my industry for many years that the biggest issue with hydraulic fluid life IS NOT the fluid itself (assuming normal operating parameters are maintained), but rather the additive packages (anti-corrosion, anti-oxidation, stabilizers, etc.) that are sensitive and easily depleted for various reasons. This is especially true in what we call “exotics” such as the ester and glycol groups, of which brake fluid is a mix of. This weakness in the additive package is pointed to in the article. The article points out a few of the typical problems when these additives are depleted. There are indeed cases where great lengths are taken to keep these packages in their proper ratios in the fluid without changing the fluid. However, what has been learned over the years is that this can only be done in vary large hydraulic systems that contain more than 500 gallons of fluid. Such systems have reached a point of economics and physical stability where keeping the additives intact is both time and cost effective. Any systems smaller than 500 gallons and it is both cheaper and more time effective to just change the fluid on occasion. Also it is important to note that when temperature conditions are high the situation is greatly exaggerated, such as around a brake caliper. Obviously, a brake system is quite small in volume and need to the fluids changed to replenish the additive package.

I won’t get into the water ratio level in detail that the article refers to. In short, there is a big question that no one knows the answer to as to what percentage that water becomes a problem in a system. That is a discussion that is a chapter or more in length. In short, I can tell you with confidence that percentages of well less than 0.5% in an ester fluid is a serious problem. We like to see in less than 0.01%. Water cases and ester fluid’s PH to change dramatically and it causes other problems. I have had one particular case study I developed a number of years ago in glass plants where very small percentages of water in their ester based fluids caused millions in damage and lost production. So why isn’t this kind of information more well known and the task force from the article just wondering about the issue back in 2002? It’s called job security. The real experts in these kind of problems make big bucks fixing them and we don’t readily just publish the info. One thing the task force in the article did realize is that the water issue in a complex one and they didn’t have the resources to delve into it any deeper. I know we have spent hundreds of thousands on study of such issues.

The one thing the article didn’t address directly was contamination by metal particles, or “fines” as we call them. Nor did it discuss metal fine contamination of a new systems when fluid is first introduced. We measure these levels in a standard known simply as “ISO contamination level”. This is the other huge contributor to hydraulic system degradation. Think of it as having dissolved lapping compound running around in your fluid. There is always some in any system. The trick is to try to get the number of particles low enough and the size of the particles small enough so that they are suspended completely in the fluid and pass through all clearances so that damage does not occur. It is an ideal obviously and cannot truly be achieved… but we can come close enough to make a system last many, many years. If metal fines are not kept to a low ISO number, they interfere with clearances which then creates more metal fines and erosion of the parts. This phenomenon of ever increasing metal fines follows an exponential curve. The build up is slow at first, but a certain point is reached where the metal fine contamination explodes and the system dies a quick death. In a standard hydraulic system, there is a filtration system that keep filtering the fines out. But in a brake system, there is no such filter, so the fines slowly build. This is why it is so important to start a new brake system with a complete flush as should be done in the production factory. All new hydraulic systems contain A LOT of contamination due to dirty tubing, new machined parts, etc. A flush is required to get all of this out and allow the system to start clean.

So what does all this mean when considering the article? At the end of all the analysis and money spent on a task force; flush your brake system from time to time to insure you have good clean fluid in your system. So nothing has changed. (sigh) We still need to flush our brake systems on a periodic basis. The only questions remaining are; How often? What defines an adequate flush? Mike was indeed right in that the only way currently to get the fluid back to new condition in a MB car equipped with SBC is to get the flush done by someone with Star Diagnostics that can properly sequence the SBC pump system and valve network to get you a near 100% flush.

Again, thanks for the articles,
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post #6 of 7 (permalink) Old 04-08-2019, 03:43 PM
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I have a copy of the WIS/ASRA and cannot find the instructions (document number) that walks me through the process, even with a STAR. Does anyone know the document number that does this? TIA
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post #7 of 7 (permalink) Old 04-08-2019, 06:59 PM
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Originally Posted by JettaRed View Post
I have a copy of the WIS/ASRA and cannot find the instructions (document number) that walks me through the process, even with a STAR. Does anyone know the document number that does this? TIA
Found it. The instructions are in document number AP42.00-P-4280R.
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