Ongoing 2006 T1N Brake problem

sbdoug01

New member
Hello,
I have posted about this before and had hoped I had been able to fix my problem with soft, nearly ineffective brakes.
Over the last year, and especially during cold weather or when driving in mountainous terrain or going up significant amounts of elevation, the brakes on my 2006 T1N 3500 become very soft, a significant difference from how they felt when I bought the van 2 years ago.
The rear brakes were professionally serviced 15K miles/16 months ago, and I have only had about 20K mi since I bought the van, which now has 120K miles, and the brake rotors and pads are in good condition..
During the last year, I replaced the master brake cylinder, and have pressure flushed the brakes many, many times, which usually results in getting a few air bubbles out especially from the left rear caliper, and in turn firmer a firmer brake pedal for 2-3 days, which then becomes weak and very spongy depending on how much elevation change or driving I do.
There is no evidence of any brake fluid leaking from any of the calipers and the brake fluid level within the reservoir is always unchanged.
I have tried to activate the ABS shortly before flushing the brakes without much improvement in the overall outcome, the brakes still get soft again over time.
I have read that in time the brushes within the ABS system become worn out.
Is it possible that this is the problem?
I have considered replacing the brake caliper, but I am not sure if that is the problem...
I would really appreciate some thoughts or suggestions on anything I may be missing or should try.
Thank you soooooo much
Doug
 

Aqua Puttana

Poly - Thread Finder
The hydraulic pressure for your normal braking is generated by your foot pressure and vacuum assist. The hydraulic pump relates to the computer controlled driving safety systems. The pump is not needed for normal braking.

There is some history of the CAB Controller Antilock Brake unit aka ABS brick allowing air into the hydraulic brake system, but not showing any external leaks. It happens *somehow*, don't ask me how. Just deactivating the ABS system is not enough to stop the problem.

Unfortunately the CAB unit is not a cheap part so many people don't believe.

I suppose that a DIY test could be to use hydraulic brake line couplers to jump out/isolate the CAB unit. I have not explored in detail the possibility of testing like that. During that testing time none of the driving safety systems would be operational.

:cheers: vic
 

sailquik

Well-known member
sbdoug01,
You wrote;
The rear brakes were professionally serviced 15K miles/16 months ago, and I have only had about 20K mi since I bought the van, which now has 120K miles, and the brake rotors and pads are in good condition."
When you say "professionally serviced" what sort of shop did the work?
A Mercedes Benz shop (dealer or independent) that has the diagnostic capability to analyze both your brakes and the ABS/ESP traction control systems?
Or somewhere else.
Until you take it to a shop that can fully exercise the ABS/ESP unit and flush all the fluid through the unit, and then run the full brake diagnostics
you may continue to have this problem. Problems with your brakes is patently unsafe. Mr. Murphy suggests that your when brakes MUST work
at maximum efficiency/best stopping power is when they will fail.
Have you done the Dr.A brake test to determine if this is just a "feel" issue, or if there is something actually wrong with the brakes/ABs/ESP systems?
Have you changed all the brake fluid in the entire system with the correct MB BEVO specified brake fluid? Done the Master cylinder/wheel cylinders/ABS/ESP unit?
Roger
 

autostaretx

Erratic Member
The "increased altitude" symptom leads me to wonder about your vacuum pump.

Since it's really trying to make a pressure differential from ambient pressure, a weak pump may not be making enough of a difference when the ambient pressure drops. (9500 feet drops you to about 11.5 psi ambient instead of the sea-level 14.7)

*added*: see Patrick's reply further down... as he says lack of vacuum would create a harder pedal.
I was thinking along the lines of "pressing as i usually press, it's not applying braking force" not "the pedal is just sinking to the floor"

--dick
 
Last edited:

Aqua Puttana

Poly - Thread Finder
A strong clue that air is entering the system.

...
During the last year, I replaced the master brake cylinder, and have pressure flushed the brakes many, many times, which usually results in getting a few air bubbles out especially from the left rear caliper, and in turn firmer a firmer brake pedal for 2-3 days, which then becomes weak and very spongy depending on how much elevation change or driving I do.
...
Doug
vic
 

SneakyAnarchistVanCamper

Reading till my eyesbleed
Inspect brake hoses/fittings for pinhole leaks. Inspect master cylinder brake line fittings and bleeder valves at the wheel are not crossthreaded or loose. Swap CAB controller with known good part, try Custom MB Sprinter junkyard in Portland for a good price. Was master cylinder replaced with an OEM part, or aftermarket? I think this would be a problem you might want to take to the best shop you know of, this can take some skill to fix.
 

lightwave

Member
" Professionals " have time constraints for a healthy profit margin. To achieve this they skip simple steps and keep their fingers crossed, or worse.
I would start with adjusting the rear brakes since that is where they were last. then master cylinder time

Good luck


___________________________________________
2006 2500 118wb standard roof 219,000 fun miles
 

Patrick of M

2005 T1N 2500 (NA spec)
" Professionals " have time constraints for a healthy profit margin. To achieve this they skip simple steps and keep their fingers crossed, or worse.
I would start with adjusting the rear brakes since that is where they were last. then master cylinder time

Good luck


___________________________________________
2006 2500 118wb standard roof 219,000 fun miles
No adjusting to be done on rears other than the parking brake adjustment which is a separate brake from the service brakes.
This is a weird problem as the system should be loosing fluid if air is turning up/unless the mc is duff or was not properly bench bled. It is impressive how hard it is to bleed modern braking /clutch systems. When I can't get pressure I pressure bleed, faster and some systems will not evacuate air properly without a pressure bleed. But yeah if that doesn't work it's time to suspect the mc, new mc can show up not working, and lead to endless head scratching.
 
Last edited:

Aqua Puttana

Poly - Thread Finder
...
This is a weird problem as the system should be loosing fluid if air is turning up ...
Bingo. The clue is that air is consistently returning after multiple bleed procedures AND no visible external leak. That is why I mentioned the CAB system up front.

My T1N's haven't been difficult to bleed using my tried and true conventional "Pump the brake honey. Hold it down... Pump 'em up. Hold... Pump 'em up... that's got it" method. When guys help me they think that my verbal instruction is a bit too intimate. :idunno:

Without being there I can't say that the CAB is 100% the problem. I can say that if the brake system is being bled properly multiple times with air always returning AND there is no external fluid leaks visible, suspect the CAB unit.

:2cents: vic
 
Last edited:

Patrick of M

2005 T1N 2500 (NA spec)
Bingo. The clue is that air is consistently returning after multiple bleed procedures AND no visible external leak. That is why I mentioned the CAB system up front.

My T1N's haven't been difficult to bleed using my tried and true conventional "Pump the brake honey. Hold it down... Pump 'em up. Hold... Pump 'em up... that's got it" method. When guys help me they think that my verbal instruction is a bit too intimate. :idunno:

Without being there I can't say that the CAB is 100% the problem. I can say that if the brake system is being bled properly multiple times with air always returning AND there is no external fluid leaks visible, suspect the CAB unit.

:2cents: vic
I agree...but if the mc is duff, the bleeding will be inadequate...leaving air in the system, and subsequent inadequate bleeding will produce a little more air, but no real improvement. Sadly I have experienced this. Anyhow replacing the mc is cheaper than the CAB. you can also test the mc by running some line out of the ports with stop fittings, bleed said lines, pedal should be hard as a rock and not move....
Last thought, I'm completely ignorant of vac assist brake servo failure. Could that introduce air in hydraulics? Seems no, but I'm throwing it out there.
 

Aqua Puttana

Poly - Thread Finder
I agree...but if the mc is duff, ...
He did mention in the OP that the Master Cylinder was replaced. He didn't mention brand or what source was used for the part.

Seems no, but I'm throwing it out there.
Certainly worth investigating. Especially if the mode is shotgun parts replacement.

I don't believe that there is any way that the vacuum assist can interface directly into the relatively high pressure hydraulics to add air.

To date I haven't seen any test method for verifying the CAB air bubble intrusion into the hydraulics problem. Unfortunately it was a process of elimination for the times I recall CAB replacement being the solution.

:cheers: vic
 

ptheland

2013 144" low top Passgr
Isn't there a diaphragm in the brake booster that separates brake fluid from vacuum? I would think that could introduce air.
I'm pretty sure that diaphragm separates low pressure air from ambient air. The difference in pressure pushes on a piston which provides the strength for the power assist.
 

Aqua Puttana

Poly - Thread Finder
I'm pretty sure that diaphragm separates low pressure air from ambient air. The difference in pressure pushes on a piston which provides the strength for the power assist.
Yep. The vacuum assist is a controlled mechanical advantage, not a direct connection to the brake high pressure hydraulics.

vic
 

sbdoug01

New member
Thanks so much, as always, to this forum's many contributors, especially Vic (aqua puttana) for the extremely helpful advice I have received regarding my ongoing brake and coolant issues with my van.
Expanding on my previous post, the brake master cylinder was replaced with a new MB part from SD Europarts. 4 days ago, the brakes were fully bled both with a pressure bleeder and with the "push-in, hold, let out, 2 person method, but they again were very soft and braking much worse within 3 days. Today, I again bled the left rear brake, and during the process, I did notice very slight brake fluid accumulation on top of the ABS unit, from where the brake lines enter and leave the unit. I tried to tight the nuts on the brake lines, but they were fully tight.
I am not sure what else to do. Vic (aqua puttana) recommended replacing the CAB unit, and that some air may be entering the unit somehow. I think he may be right. and that somehow there is some air entering the brake line from the ABS unit somehow, but it may not be enough air entering the lines to change the level of fluid in the reservoir noticeably.
I guess I don't really understand how the CAB unit, which I would think is sealed, would allow air to enter into the brake lines, while still functioning without any problems and without any warning lights showing up on the dashboard?
Does anyone know where to buy a replacement CAB unit at a reasonable price, and how to make sure I don't just buy another defective unit?
Thanks again, so much.
Cheers,
Doug
 

sbdoug01

New member
Bingo. The clue is that air is consistently returning after multiple bleed procedures AND no visible external leak. That is why I mentioned the CAB system up front.

Without being there I can't say that the CAB is 100% the problem. I can say that if the brake system is being bled properly multiple times with air always returning AND there is no external fluid leaks visible, suspect the CAB unit.

:2cents: vic
Hey Vic,
Thanks again for your help. I do think that somehow a small amount of air is coming into the brake lines. I guess I don't really understand how that is possibly a CAB unit problem, since my understanding of the CAB and ABS unit is that the CAB unit is not in direct contact with brake fluid. It would seem to me that if there is some problem with air leaking into the system, that the machined aluminum ABS unit is likely to have a leaking seal, somehow allowing air into, but not out of, the system?
If that were the case, since brake fluid is under a lot of pressure, while outside air is unpressurized, wouldn't the fluid leak out, rather than air leak in?
Should I take the entire ABS unit out and attempt to seal it completely? Would some sort of liquid gasket be sufficient, you think?

Doug :cheers:
 

SneakyAnarchistVanCamper

Reading till my eyesbleed
brakes were fully bled both with a pressure bleeder and with the "push-in, hold, let out, 2 person method, but they again were very soft and braking much worse within 3 days
AFAIK you need a scanner to actuate the ABS pump to properly bleed a sprinter. Perhaps you just had a lot of air in the system from recently replacing the master cylinder, and are unable to get the last bit of air out without a scanner? Worth a shot bleeding the snot out of it, with a scanner, before replacing the CAB; jmo.

I did notice very slight brake fluid accumulation on top of the ABS unit, from where the brake lines enter and leave the unit. I tried to tight the nuts on the brake lines, but they were fully tight.
That right there is what I would focus my attention on. If it's leaking, it can also introduce air. You could drain the reservoir, unscrew the fitting, inspect for debris or damage, screw the line back in and refill. If the leak reappears, I would replace the unit. This is where it is leaking, on top?:
 

Attachments

Last edited:

Aqua Puttana

Poly - Thread Finder
Hey Vic,
Thanks again for your help. I do think that somehow a small amount of air is coming into the brake lines. I guess I don't really understand how that is possibly a CAB unit problem, since my understanding of the CAB and ABS unit is that the CAB unit is not in direct contact with brake fluid.
The CAB unit is most certainly in contact with the hydraulic fluid. It pumps/directs the fluid for ESP/ASR system actions and uses solenoid controls for the ABS functions.


It would seem to me that if there is some problem with air leaking into the system, that the machined aluminum ABS unit is likely to have a leaking seal, somehow allowing air into, but not out of, the system?
My guess is that it is some sort of seal which lets the air in. I would expect a flaw in the casting (crack, porous) to leak fluid out fairly consistently.

If that were the case, since brake fluid is under a lot of pressure, while outside air is unpressurized, wouldn't the fluid leak out, rather than air leak in?
The brake hydraulics are under pressure only when the fluid is put under pressure by your foot/vacuum assist or the CAB pump. At all other times the pressures go to zero and allow the brake pads to relax to slightly back away from the rotors to not drag.

Air is under atmospheric pressure. My guess is that the air gets in from changing barometric pressures, changes of ambient temperature, or changes of altitude, or a combination of all three.

Should I take the entire ABS unit out and attempt to seal it completely? Would some sort of liquid gasket be sufficient, you think?

Doug :cheers:
That might work for a crack or porous aluminum. As I said earlier, that failure mode would have visible leaks. You do have some visible fluid so maybe a porous casting... , but I don't believe an attempt to generally seal the unit would be successful.

There are seal designs which hold pressure one direction better than others. So while the CAB unit is under pressure a wonky seal can hold fluid in. While relaxed and not under pressure it could let air under barometric pressure in from the opposite direction. I suspect that may be the CAB air intrusion failure mode.

I had a similar coolant system situation with a V6 Utilimaster walk in van. Radiator non-reservoir system, unvented cap. There was a high point bleed on the system. The engine would start to overheat. Bleeding the high point air out always worked for a time. Never any external leaks or fluid loss.

After trying different things, I finally decided to change to a reservoir with vented radiator cap. With the OEM unvented cap and no reservoir, when the heated engine dropped temperature a vacuum was created. That would pull air in from a seal somewhere. The seal never leaked under pressure. The vented cap and reservoir system cured the problems completely.

The CAB unit main passages are definitely being included while using conventional bleed methods. The scan tool pump activation causes any fluid trapped in solenoid circuits and the pump reservoir to be released and renewed. I would be surprised if a scan tool activation type bleed process will make a difference with the air intrusion problem. Certainly it is inexpensive and worth a try. The air is getting in from somewhere, not residing within the system.

To be clear, I am not saying that the CAB unit is definitely your problem. There is some history. You do mention brake fluid showing on the CAB unit and not anywhere else.

:cheers: vic

Added:
...

I suppose that a DIY test could be to use hydraulic brake line couplers to jump out/isolate the CAB unit. I have not explored in detail the possibility of testing like that. During that testing time none of the driving safety systems would be operational.

:cheers: vic
 
Last edited:

calbiker

Well-known member
I would replace the left rear caliber. It doesn't hurt to replace the small section of rubber hose either.

I have noticed while bleeding brakes that it's easy for air to get sucked through the bleeder screw. I know your bleeder screw is tight, but it may have enough corrosion to allow a small amount of air into the lines.
 

Top Bottom