Back in the day I would increase the pressure a bit too over what the label said, for various reasons:
The car was driven to the shop, which warms the tires, the tire pessures are given for cold tires, so they probably should be increased a bit over the cold pressure. This is assuming I knew the car was just driven in and was in the shop pretty fast after it got in.
Second, the pressures are for a normal load, if the customer loads the car up with clients or what have you, it would be a benefit to have the pressure slightly higher. (Second part A) Also MB recommends higher pressure for higher speeds, so they are also covered if they develop a lead foot for awhile (late for work, cops chasing you, etc)
Third, the fuel mileage may be a bit better.
Fourth I think they handled a bit better that way.
Gilly

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At great risk to myself, since it may appear that I'm being disrespectful to you by pointing this out (believe me, that is NOT my intent), I couldn't help myself.

As a street rodder as well as Mercedes owner, I spent a lot of time studying brake systems and components and feel the need to clarify what this valve's job is.

The best explanation of the proportioning valve I've found is right from the Howstuffworks website:

"The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.
The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.
If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force."

I don't know if there is one in the Mercedes system or not, since it is all-wheel disc and as you note, the valve is usually associated with front disc/rear drum systems. And I'm neither a professional mechanic or as knowledgeable a senior poster as yourself, so please accept this observation as an attempt to clarify the problem posted by the OP.

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Hmmm, my 2000 Corvette has 4-wheel discs, and a proportioning valve.
It actually changes the percentage of force between the F/R as the pedal pressure is increased ( at light pedal, it might be 60%-40%, and heavy pedal it might be 80-20). This variable ratio can be modified by changing springs in the proportioning valve, to bias more or less % to the rear at higher or lower pedal pressure.
Makes sense, I guess; I would assume at light pedal the weight transfer is less, and the rear could use a bit more pressure. At full panic pedal, the weight transfer is greater, and the rear need less percentage of the brake pressure. Of course at full pedal pressure, each wheel is controlled by the ABS, so it's moot at that level. Sure stops good though - and so does my 300SE (amazingly so for a 2-ton hog).

Considering tire pressure, someone awhile back commented that no matter what car or tire you drive, it's hard to be too far off with 32 psi.
He may be right, I found the best pressure for wear, ride, handling on my Vette, my Benz, and my PT Cruiser is right around 32psi for all of them.

DG

I think it's cool that inside the fuel door are posted tire pressures... But what really impressed me is the need to add a few pounds if driving over 100MPH. Time to test those tires up I-5. hehehehe

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Sure sure, 2000 Vette, probably slightly modified brakes that they used on a 1999 Lumina APV van........
Gilly

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hey, no porblem. i have no godlike wisdom here, just 40 years of tinkering and reading about cars. your post is a good definition in general terms and is not wrong. mercedes seldom needs them for several reasons. first, they do a pretty nice job of keeping their cars close to 5050 weight distribution front to rear, unlike some manufacturers who will have engines of vastly different weights. so that reduces the need. and second..... cant think of another point.

the post about the corvette valve is very interesting. i would have thought something like that unneccessary as the true need for it would be mainly at the limit of adhesion...but in a vette wet pavement will make it vastly different.

i think also pickup trucks may have some variation of a variable prop valve too to account for the vast differences in braking needed when empty or full.

some good discussion here of some basic engineering principles that are very important.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

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