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2004 E320 SBC reservoir pressure decreased after changing brake fluid
Hello,
My car is a 2004 E320 sedan. For several years I checked with my DAS Xentry C4 the pretensening pressure of the pressure reservoir was always 84 bar. One week ago I had my car changed the brake fluid by an auto shop who specializes in Mercedes cars and now the reservoir pressure is only 77 bar. Could you please help me if this reduction in pressure is caused by old pressure reservoir or old SBC pump or the mechanics did not properly apply correct pressure when changing the brake fluid?
Many thanks!
Vietace 77 bar is the new pressure and 84 bar was before the brake fluid was changed.
77 bar is still in acceptable range, though it's on the lower side. Mechanic might haven't looked the pressure beforehand, and just set it to range. Does it make difference if it's 7 bar lower than before when it's in specs? Did you specify it to be at 84 bar when you took the car to service?
I didn't know that it's possible to specify the pressure at 84 bar. Could you show me how to set it in DAS so that I can explain to the mechanic? So far I don't feel any difference when driving, however, I would prefer the pressure more than 80 bar just to feel safe as this is the SBC. Thank you.
Unfortunately, I'm not familiar with the DAS, nor with SBC brakes, either. My comment was based on basics of hydraulic systems. If there's a pressure reservoir, you should be able to set the pressure where ever you want with the correct tooling, and also my personal experience with customers. Some are very specific on adjustments, some are happy if it's in specs. When in doubt, we usually ask what they want it to be set in.
Go talk to the mechanic who did the job, and tell that you'd feel safer if the pressure is above 80 bar.
The SBC pretensioning pressure reservoir is a sealed unit. The pressure in it is what it is and cannot be modified in any way; it is pre-charged and then sealed and then assembled to the SBC assembly.
For what it is worth, the pretensioning pressure reservoir in my E55 sat at 70 bar for years before it finally triggered an error code and the white visit workshop indicator on the instrument cluster. At that time I took it into the Mercedes dealer and they replaced the pretensioning pressure reservoir at no charge to me in accordance with the 25-year extended warranty on the SBC system.
Did your mechanic use the Star/Xentry diagnostic system to flush and bleed the SBC brake system? If he did not, there is a possibility there is still a bit of air somewhere in the system that makes it appear that the pretensioning reservoir is low on pressure when it is not. If he did use a Star/Xentry system to flush and bleed the SBC system, I really do not have a definitive answer for you as to why the pressure would have dropped by seven bar. For what it's worth, in one area of the diagnostic system it shows minimum pressure for the pressure reservoir to be 72 bar while in another area of the diagnostic system it shows the minimum pressure to be 70 bar. Yours still seems to be doing pretty good.
Perform in advance search under my name and you should be able to find a very lengthy technical documentation package that was used to familiarize Mercedes-Benz technicians with SBC system when it came out almost 20 years ago.
Last edited by bbirdwell; Aug 6, 2022 at 02:38 PM.
The SBC pretensioning pressure reservoir is a sealed unit. The pressure in it is what it is and cannot be modified in any way; it is pre-charged and then sealed and then assembled to the SBC assembly.
Ok, so it's kind of like what GM had on their Hydroboost brake boosters.
And thank you for clarification, I really didn't know what I was talking about
Could you please help me if this reduction in pressure is caused by old pressure reservoir or old SBC pump or the mechanics did not properly apply correct pressure when changing the brake fluid?
Thank you all especially bbirdwell and konigstiger for your very helpful information. My mechanic did use the Star/Xentry diagnostic system to flush and bleed the SBC brake system. He even flushed and bleeded the SBC system twice. I drove a 155 mile trip yesterday and the SBC brake system seemed working fine. Before the trip I checked the reservoir pressure is 76 bar, one bar lower than last week. I will keep checking the pressure before I drive. I may replace the pressure reservoir as I already bought one months ago. I don't live in the US so my car doesn't have 25-year extended warranty on the SBC system. bbirdwell I have read several very informative writings about the SBC brake system you wrote here in this forum. I will read the SBC documentation package you posted here.
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