rx7turbo2

I tried to avoid the word "faulty" I don't believe that's the right term. I used "weaker". Maybe I can describe it better. A small number of brake boosters have a more sensitive threshold of vacuum needed for 100% proper operation. Perhaps that's because the diaphragm is stiffer first thing in the morning, perhaps the return spring is slightly stiffer, I'm not sure. That may be the reason Mercedes decided to patch it. They already had the electric vacuum kit available because it comes stock on a number of the other W203 models so they figured why risk replacing the brake booster to find it doesn't solve the issue when we can just put the retrofit kit on and be done with it.

I have tried as you suggested. Vehicle at normal operating temperature, took the vehicle for a drive to confirm proper brake operation. Got home and removed the vacuum line to the brake booster. Then put the car in reverse and backed down the driveway. The braking sensation was very similar to how I describe it when the vehicle has issues from a cold start. It was more exaggerated however. Makes me think that when I'm having issues from a cold start I'm at least getting partial vacuum assist not full assist. Which leads us back to the likely cause of either A) For one reason or another there is slightly less vacuum at idle (I'll try and verify with actual numbers) or B) Something in the brake booster is "stiff" or "binds" after it sits overnight. Perhaps a combination of both I suppose.

Glyn M Ruck

The fact that it holds vacuum overnight tells us we don't have a leak post NRV.

I think the best explanation is that the booster is sub par for whatever reason.

The further you open the throttle, initially the lower the vacuum. I find it hard to believe that high idle cold is sufficient.

I think we are getting to decision time. Band aid the darn thing or recon/replace the booster. You can check the booster ambient air filter & valve. I'm not very hopeful though.

Either choice is going to cost you money & we are all going to be pissed if it does not fix the problem. We don't have the dealer pleasure of being able to "try out" new parts & return to stock if it does not work.

Glyn M Ruck

BTW ~ this issue has made up my mind on the Jag I'm restoring. I have no option but to re-sleeve the calipers in stainless & turn new SS pistons. New calipers are not available & this is going to be a concours contender. The rest I'm replacing with new. Lines, unions, booster, master cylinder & SS braided flexibles.

rx7turbo2

I'd like to get my hands on a vacuum gauge just to verify if there is in fact a difference between cold idle and hot idle vacuum. And if there is what sort of differential we're looking at. Any idea's what sort of vacuum reading I should expect from this engine at 4000'?

We have a very good relationship with our local Mercedes Service Center, I'll bring all the info to them and have them bring it to the attention of Mercedes Canada, I think it's unlikely but you never know perhaps they'll be willing to work with me a bit when it comes to the cost of either replacing the booster, or installing the retrofit electric pump. My guess is Mercedes won't do much, but its worth a try. If in fact they tell me I'm on my own I think I'll take the following approach. Since the most recent flush/bleed it's been much better. There is no longer a risk of the vehicle rolling uncontrolled across the sidewalk and into the street. Yes it takes what I consider more pedal effort than should be necessary to stop the vehicle initially after a cold start, but I no longer consider it a major safety concern. I'll monitor it going forward and if it becomes a major nuisance I'll install myself, one of the many other electric vacuum pump options available at a considerably better price than Mercedes want's for theirs.

Glyn when it comes to your Jag I think you're bang on. You know better than most when it comes to hydraulic braking systems that use pneumatic assistance sometimes you can spend a considerable amount of time chasing your tail when a problem arises, as this thread can attest to. Since new calipers for the Jag aren't an option you'll probably save yourself a major headache by re-sleeving them now and machining a set of pistons to match. Cost more? I would think so, but long term you'll have faith in the system and reduce your risk of encountering an issue to just about zero.

Glyn M Ruck

At idle from memory I would expect about -80 and -70 kPa or 20-22 in Hg with the booster as the only consumer. These are a rough idea. Ask Benz. I have no idea at 4000RPM

bop11

Is this a cold weather phenomena? Did you look at the other end of the booster, the pedal end?. It works because normal air pressure pushes against the non-vacuum side. As you remember, there is no such thing as a vacuum only a pressure lower than atmosphere. One side of the diaphragm is connected to low pressure but if the other side is not connected to atmosphere you don't get any brake boost. In older cars, it was not uncommon for the high pressure side to be around the rod connected to the pedal. This port had some fuzzy filter material which could get wet or oily or dirty or even freeze keeping the normal air pressure from the high side of the diaphragm. With some use or cab heat, the air could flow into the high pressure side.
If water or oil has gotten into the booster, possibly through the air side, then the valve that connects the two sides of the diaphragm, internal to the booster, may have a problem closing and again you don't build high pressure on the hp side. Do you suspect that the car was ever in high water

Glyn M Ruck

Indeed!

rx7turbo2

Very good point about the booster and how it requires both low and high pressure and the diffferential to function!

I have focused my attention on the "low" side and neglected the "high" side. Does anyone have a picture of the ambient air filter and valve and where they can be found?

As mentioned earlier the vehicle has led a pretty easy life, no flooding damage or anything like that, but it's possible the valve and/or filter is sticking or plugged, I'll check it out.

Glyn M Ruck

I've got a horrible feeling that this is a trick question & it's something we have not covered in the Wiki yet. I think that there is something odd about this booster. I've just pulled a picture from the internet. There is no external white plastic filter cover & valve like most conventional boosters held on by about 6 screws. It's either internal or in the face of the booster.

On the face of the booster you have the master cylinder (central), the daiphragm position sensor for the ABS/BAS (Tubular protrusion with connector), the vacuum pipe attachment grommet & another point that I can't identify ~ maybe that is it but I don't know ~ it is not listed in the EPC & I don't know how it would access the ambient side of the diaphragm, internally obviously. Maybe someone else can chip in. I've never had a booster failure on one of my cars.

I will continue to scratch. My laptop with WIS loaded has died.

VVF

Here you go.
Both are connectors.

Do you think it's much different conceptually from what's explained in this video?
Power Brakes - Vacuum Assist - Explained - YouTube

Glyn M Ruck

Hah ~ got it. Both filters/silencer & valve are under the input shaft boot. You have to pull the booster.

Glyn M Ruck

Thanks VVF ~ I'll get those into the Wiki. I need to sort out my WIS.

rx7turbo2

Thanks guy's!!!

I was afraid they'd be up in there like that. If I've got to pull the booster anyways to get at them, I'd be inclined to just throw a new booster in after the old one is out.

As the temperature here begins to get colder by the day I'm going to monitor if that seems to play a factor.

Glyn M Ruck

You can't buy seals, filters etc anyway for that input shaft. You have to buy a booster.

Glyn M Ruck

ABS & BAS have certainly complicated the good old booster but a picture paints a thousand words.

VVF

I hate how one cannot buy some things to be able to rebuild parts... $5-10 vs the new booster...

bop11

Wow, I had forgotten how complex these are now with pulsed brakes and the solenoid that automatically increases pressure if the accelerometers think that a crash is imminent.
In the diagram, g is the bellows filter and f is the high pressure side filter. However, after seeing the diagram, I would guess that there is a substance causing the BAS solenoid y1 to perform sluggishly until operated a few times. If this fails to open the port, then you are basically in a brake boost failure and relying on direct pedal to rod to master cylinder to apply hydraulic pressure to the brakes. This was OK with drums as they are somewhat self activating but needs a very heavy foot with disks.

VVF

I think you are partially right.
WIS says it operates like a standard brake booster, and only during emergency braking does the solenoid open the valve to let the atmo pressure in, reaching maximum boost instantly.
I would expect that if you press the pedal slowly but to the floor, you would get the same max boost (i.e. equal to the ratio of atmospheric pressure vs. the vacuum).

rx7turbo2

I've spent some time reviewing the BAS system, a very interesting system, and kind of genius when it first came out that's for sure. It did certainly complicate what had been up until that point a relatively simple device

I'm pretty confident this car has suffered this issue since new, as there is some evidence that it wasn't the only one. Now that the bleed/flush has resolved it enough that I don't believe it's a major safety issue I'm inclined to leave it. That first few feet in the morning require slightly more pedal pressure to stop the car than is required when the car is warm, but the difference now is marginal. I'll monitor it, and if it gets any worse or I'm feeling industrious maybe I'll install a new booster, or as Mercedes did "band aid" the issue with an electric vacuum pump.

rx7turbo2

***Update***

Okay, finally had a chance to throw a gauge on the car to see what is going on.

Scenario 1
-Car is at operating temperature. Vehicle is in park, vacuum at the brake booster reads 15-16inHg.
-Car still at operating temperature. Vehicle placed in gear (drive or reverse) vacuum drops to 12-13inHg.

Scenario 2
-Car is cold having sat overnight. Vehicle in Park, vacuum reads 10inHg.
-Car still cold. Vehicle placed in gear (drive or reverse) vacuum drops to 6inHg.

Under both scenarios the RPM is what you would expect it to be, no abnormalities in that regard

When at operating temperature as mentioned previously in this thread the vehicle braking system operates as you'd expect it should, and the numbers basically support that. While the 16inHg isn't high it's sufficient to assist the braking system via the booster I think.

However the low vacuum during that first cold start of the morning is almost certainly the issue at hand here. It also confirms the previously posted TSB regarding this issue. That low vacuum at cold idle only lasts a very short period of time. If the car is reversed down the driveway, by the time I get to the end If I change directions to come back up the driveway the vacuum has already started to improve, and in a very short period of time it's improved to where it is when it's at operating temperature.

The million dollar question is why is the vacuum low during that very specific and short period of time? I have no theories, and the fact Mercedes "patched" it with a "bandaid" fix of an electric pump leads me to believe they don't know either.

Anyways I think ultimately I can rule out the master cylinder, and after all the tests the booster as well. The low vacuum is whats causing this issue. Again it really is a minor inconvenience, but had me stumped. I think if I decide to fix it for good, I'll probably do what Mercedes did and just add an electric vacuum pump. Unless anybody has a theory on what's causing the low vacuum at cold idle?

slagdemon

My car does the same thing.. 2003 C240 RWD. My driveway is small but steep and I really have to mash the brake to get the car to stop at the bottom of the drive. After I go half a block or so, the brakes are normal. I'm going to check the vacuum line and valve to see if I can find anything out of the ordinary.

rx7turbo2

slagdemon

Thanks for the info. I'll just live with it. It's not worth spending the large amount of money to fix a problem that exists for 10-15 seconds.