rx7turbo2

Having a strange issue with the braking system on my 2003 C320 4Matic.

I've searched but can't seem to find anyone that's had the same issue.

In the morning (after the car has sat overnight) when backing (reversing) out of the garage and down the very slight slope of my driveway the brakes "feel" weak. Sometimes you can push the brake pedal to the floor and the vehicle will stop but not with that sudden sharp jab that you would expect from putting the brake to the floor. It will kind of roll to a stop, sometimes that roll is longer than others. While doing this the brakes also have a very light squeal, sounds almost like the pads are engaging the rotor but not fully?

If you're doing lets say 10-20kms/hr (forward direction) the same thing occurs. Panic brake and jab the brake to the floor and instead of a sudden and violent stop that you would expect, the car will roll to a stop, now it rolls to a stop fairly quickly but I'm used to cars that will lock the tires up and engage ABS under this condition, this car does not.

The vehicle has 70,000kms on it. The brake pads and rotors are original and have lots of material left. The brake system has been bleed properly numerous times I'm confident there is no air in the lines. The rest of the vehicles driving behavior is just fine, including the braking. Pedal does not sink to the floor, pedal has good feel, pumping the brakes makes no difference, no error codes etc, it's just that first thing in the morning reversing, and panic braking in the forward direction. The only other sign is that the front wheels collect considerably more brake dust that the rears. I've never owned one of these cars so I don't know if that's normal, but the front wheels will be black while the rears have no dust. Is it possible that whatever controls the brake bias has failed? Causing the fronts to compensate for the lack of rear braking?

Any idea's? I'm stumped, I don't want to just start throwing parts at this thing in the hopes I'll get lucky.

Thanks

Glyn M Ruck

The Dust is normal at the front. I suspect you have a booster problem. Let's hope you don't have an ABS block problem ~ that get's expensive. Check the vacuum line to the booster for blockage or leaks & report.

If you hold your foot on the brakes & then start the car. Do you feel the pedal soften slightly & then firm? If so this is normal & correct.

With Stock pads sharp braking should immediately engage the ABS & drag the car to an ABS induced juddering halt.

Good luck!

VVF

So this is your second incarnation?
https://mbworld.org/forums/c-class-w...3-c320-4m.html
Or is there something happening in Canada that causes brake failures in 2003 c320 4Matics? O_o

rx7turbo2

Thanks Glyn!

I forgot to mention this in the initial post. With the car not running (key not even in the ignition) if I pump the brake pedal it gets hard, then while still keeping my foot on the brake pedal if I then start the car the pedal will sink slightly and get firm, does this rule the booster out? Even when I've done this procedure then backed out of the garage down the slight slope it still acts in the way I first described.

The ABS will engage at faster speeds. If I was doing say 50km/h and panic braked it would engage the ABS as normal, when road conditions are wet and/or snowy the ABS functions as it should as well.

Would glazed rear pads or rotors cause this?

rx7turbo2

Sorry. Didn't see that thread

That's my father posting, didn't know he was a poster here. He and I have been racking our brains trying to get to the bottom of the issue. If a mod is reading feel free to amalgamate the two threads.

Glyn M Ruck

It sounds as thought the booster is at least partially behaving. Might still not be providing the correct ratio of boost.

Yes the ABS would very much engage a 50 km/h

This is unlikely a glazing issue. Rear brakes do very little which is why you get so much dust up front on these cars with stock pads.

tommy

Texans are so untrusting.

Good luck with the brakes.

rx7turbo2

Your assessment of the rear pads was my understanding as well. Whenever you see these cars the front wheels seem to be covered in brake dust.

If the Booster is to be replaced is it best to stick with OEM? If the Booster is changed should the Master Cylinder be done at the same time, one of those "while you have it apart" scenarios?

rx7turbo2

I'm from Alberta, Canada's Texas, I completely understand

VVF

Lol, I am from Russia originally.
Actually I saw that you only had 1 post and thought that you lost the password to your other account or something

Is it possible to disconnect the vacuum hose from the booster to check that there is indeed vacuum in the hose?

Glyn, why is it that the brakes are so much biased toward the front? Does that mean that the rears are there pretty much for ESP etc?

rx7turbo2

Update.

The system was flushed at the dealer. Pedal feel was considerably better afterwards. I previously described that at 20km/h if you panic stop and jump on the brakes it would stop quickly but was not capable of actually locking the wheels up and engaging ABS. That is no longer the case. Now (after flush) 20km/h panic stop, stomp on the brake pedal and the tires will lock up and the antilock system engages. We'll see how it is in the morning since that's when the issue seems to be most prominent.

VVF

Fingers crossed.

How much does the dealer quote for a brake fluid flush? I believe an indie shop quoted around 150 when I asked.

rx7turbo2

That's on par with what the dealer charged me. I think it was $160(CDN) give or take.

Glyn M Ruck

Interesting ~ let us know how it goes. Has this car had it's brake fluid changed every 2 years?

rx7turbo2

This morning was back to the same. Yesterday when vehicle is at operating temperature everything functions as you'd expect, no issues whatsoever. Vehicle sits for 12 or so hours, ambient air temperature is now 5degrees(Celsius) and the issue arises again. In reverse backing down very slight slope very slowly requires far too much pressure on the brake pedal to stop, slam the brakes on and the vehicle glides to a gentle stop.

I'm trying to determine what possible components are affected by either the change in temp, the sitting motionless, or both, but I can't think of anything in the braking system that these variables should affect.

Glyn I'm not sure if the fluid has been changed every 2 years since new as the vehicle was purchased used. It has now however received 2 flushes in a short period of time. One interesting thing to note however is that the Tech who did the most recent flush noted the fluid was "dirty" Which I thought odd since the flush that occurred previously was preformed only a couple months prior. If the vehicle had a lack of flushes prior to our ownership could that cause damage to the seals of say the master cylinder? I have a basic understanding of how the tandem master cylinder works on these cars. Is it possible that while the vehicle sits, worn seals in the master cylinder are allowing fluid to migrate?

As I originally mentioned I'd rather not just start throwing parts at the issue, but it's such a strange scenario I'm not entirely sure where to go from here lol.

Glyn M Ruck

I would still be making sure that the booster is getting full vacuum (no dirt or leaks in the vacuum line) & that there is no bellows/diaphragm leak in the booster.

The biggest danger with ABS brakes & dirt/corrosion by-producrs in the fluid due to water build up (brake fluid is hygroscopic) is that the ABS unit has a pump that circulates the fluid through the ABS block. The ABS block is a complicated piece of kit with numerous valves, line dampers/accumulators & very fine passageways. Dirt in the fluid can do big damage to the ABS block.

I doubt you have a master cylinder problem. If you did you would either have fluid leaking through the firewall onto the drivers side carpet or into the booster & possibly out of the exhaust. Does the vehicle use any brake fluid other than occasional slight top up to compensate for pad wear?

rx7turbo2

An issue with the ABS block would be bad news, I know they're pretty pricey to replace.

I suppose it could be an issue with the diaphragm in the booster sticking or binding after its sat overnight? You'd think if it was a vacuum leak it would be an issue all the time not just after the car has sat for awhile? Is there a check valve or anything in that vacuum line to the booster that might be sticking closed? I've never done the basic booster check first thing in the morning, I'll try that next.

Fluid level has always been fine, and no signs of external leakage.

I may not have a choice but to ultimately replace the booster, and might as well do the master cylinder at the same time and cross my fingers. Guess I have to decide how far down the rabbit hole I want to go.

Appreciate the expertise Glyn, I'll update the thread as I find out more.

rx7turbo2

When it comes to the vacuum pressure for the booster is it strictly manifold vacuum pressure assisting the booster? Or is there an electric vacuum pump that also assists?

Glyn M Ruck

Just Manifold vacuum.

I have been looking back at records that I should not have & you are not the first to have this issue.

Found this for an early just pre facelift 2005 C320.

"LI-43-10 08/03/2010 10034988 Mercedes benz (daimler) mercedes-benz-c320-2005: unsatisfactory brake servo assistance after a cold start, primarily when maneuvering. low vacuum in the intake manifold due to wide open throttle valve. retrofit electric brake booster vacuum pump (m56). model 20 2011-02-04"

Now this is a band aid & I do not understand the WOT comment. There are no ways that the throttle would be wide open. They should have diagnosed the lack of vacuum. I had a M112 powered 2003 C240 2.6l. Basically the same engine but lower capacity & there was no lack of vacuum.

There is a NRValve in the booster vacuum line. I have seen those vacuum lines get very clogged due to the captive breathers on these cars. In fact the variable inlet manifold & flaps can get very fouled up.

rx7turbo2

Through some digging I found two interesting documents.

The first is a Star Bulletin

http://www.google.ca/url?sa=t&rct=j&...,d.aWM&cad=rja

The second is explaining the retrofit for the pump mentioned in the bulletin.

http://mercedesbenz.workshop-manuals...x.php?id=10132

Thoughts?

***Edit*** Glyn you and I must have dug in the same area

Glyn M Ruck

I think the Service Tech report I found was quoting what you found & erroneously embellishing it in his job card report. Benz would never pass the WOT comment.

However it begs the question. Are a very few cars for whatever reason subject to low manifold vacuum at cold start open loop, high idle while warming the Cats? I find it very hard to believe because most vehicles do not suffer this problem. The M271 powered W203's have a vacuum pump & so do the diesels because a diesel does not produce adequate manifold vacuum.

My old Jag gets around this problem with a vacuum tank ~ simple solution. But why do all M112 powered W203's not suffer this problem. Makes no sense.

Fitting an engine driven or electric vacuum pump is a work around rather than a proper diagnosis. I would rather understand why it happens on very few cars but it is an option.

What cold RPM does your car idle at after start?

I wonder if there are a few boosters out there that are sub par.

rx7turbo2

The retrofit electric vacuum pump fix supplied and installed by Mercedes is pricey, $2k+. I agree Glyn it's a "band aid" or a "patch" it really doesn't address the underlying cause. It's possible this car has suffered this issue it's entire life. It's only got 70k kms on the odometer and all of them would have been pretty easy. As mentioned 99.9% of the time the braking of this car is spot on, it's very possible nobody noticed the issue until me.

Keeping in mind the car was at operating temperature, I managed to remove the vacuum line to the brake booster, I don't have a gauge, but the vacuum felt very strong. I will check it tomorrow on a cold start to compare. The line as well as the check valve appeared very clean no debris whatsoever. I'll check the idle RPM when I do a cold start tomorrow.

I'm not real keen on "patching" the issue I'd like to get to the bottom of it. Having said that if a retrofit electric vacuum pump solves the issue, I'm confident I can install a system that will work well for a lot cheaper than Mercedes want's for theirs. I have experience installing similar systems on Hot Rods and ported Rotary engines.

Glyn M Ruck

OK ~ so you have established good vacuum & we have to remember that this car has low mileage ~ pleased the vacuum tube is not fouled. It would be interesting to check vacuum at a cold start. I can't believe it is bad at high idle however. The throttle plate is simply not wide enough open.

IMHO it seems likely that the brake booster is faulty. It might have been like that from new. I do not understand how it improved briefly after a brake fluid flush however. This is a very interesting head scratch. It also amazes me that Benz adopted a band aid fix in a bulletin. Why did they not replace the booster.

Benz has been known to be wrong. You might have found the SA W204 case thread I was involved in where the brake pedal would go to the floor & they refused to believe it was the ABS unit. Well they were wrong. It was the ABS unit.

rx7turbo2

Tested the vacuum at idle during a cold start. Two notes. The first is that when I removed the NRValve from the brake booster it was clear there was a considerable amount of vacuum in the booster, since the car sat overnight it probably rules out a leak in the booster since it was capable of holding vacuum for that period of time. The second is that the vacuum felt strong when the vehicle was idling cold, as strong as it did when tested during a hot start? Tough to say since I don't have a gauge and am going by feel. I would say it might have been slightly lower than when tested during a hot start, but not by much. I'm going to try and get my hands on an actual vacuum gauge so I can get an accurate measurement to compare. In addition the cold idle up seems reasonable. when first started RPM's climb to 1100-1200 for just the first split second then return to 850-900 and hold there steady, this is while the vehicle is in park, they of course drop slightly when the vehicle is put in gear.

I'm beginning to think you're right. Some of the boosters in the w203 model line up while not faulty as far as having a leak is concerned may be slightly "weaker" for lack of a better term. Some of the models (the Kompressors, and diesels) where given electric pumps to compensate for the fact they produce less vacuum at idle. However on some of the other models it only manifests itself when conditions are just right. I for instance have one of the boosters that is "weaker", I have one of the models that does not have the electric vacuum pump, and I live at 4000' elevation.

As for why it got better after a dealer preformed brake flush/bleed. I have no doubt that the flush/bleed preformed before the dealer one, left air in the system, also that flush was done at sea level, then the car was brought here to elevation, probably making the effects of air in the system worse. The air in the system acerbated the issue and is what really brought the issue to light. It made me hyper sensitive to the situation. It's part of the reason I believe none of the previous owners noticed the issue. Truth be told, had air not been left in the system after that initial flush I may not have really noticed either. The dealer flush/bleed solved the air in the system issue, and improved the situation considerably but like I said I was now hyper aware.

Now why did Benz "patch" the issue, instead of fixing the underlying cause? I find that strange as well to say the least. But as you said they're not always right.

I stumbled across that thread while researching the issues I was having. Very interesting where it went. Certainly took time to get Mercedes to do anything about it

VVF

I am starting to think it's not the vacuum...
If it still had vacuum after sitting overnight, what makes you feel it's faulty?
Maybe a valve is sticking in the booster that lets the ambient air enter the chamber, and with several brake applications it kind of frees itself? just a thought.

Did you try to simulate an overnight leak like so: on a warmed-up engine when the booster is working fine, (maybe after you return home in the evening) disconnect the vacuum hose, then reconnect again and see if it behaves the same as when backing down the slope on cold start.