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.
THE BAD NEWS:
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.
THE GOOD NEWS:
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.