Another W126 brake question
 

The brakes on this coupe seem to stop you on a dime with little foot pressure. Unlike my Jeep. However when you come to complete stop and with foot firmly on brake pedal, the car seems to waddle a little front to back. This is not a suspension thing. Looking out the door as I come to a stop and down at the tire the waddle seems to be the weight of the car rocking on a cushion of air(the tires). Like the tires are not firm enough (44PSI) to support the car. I have wondered perhaps that only the front brakes are working. So yesterday I decided to try something different. Since I will be getting new tires soon, I did a panic stop at about 40MPH. The front tires locked up. But not the back ones.
I have a vacation coming in a couple of weeks. So I will have extra time to hover over this issue. I have a new radiator,fuel pump,and accumulator to put in too.
Am I looking at a master cylinder problem,booster, or simply a bleed. The pedal is not spongy in any way. I feel as though I have all the braking power I will ever need.
Any suggestion greatly appreciated:kid:

i had such a problem with my sec.

it was caused by replacing the front pads with a different brand then the rear. i changed the rear to match the front and now have no problem.

i had the ceramics from tire rack on front and mb factory on the rear. the fronts would lock on wet pavement or snow and even eventually on dry if braking hard.

i bought some for the rear to match and the problem is gone.

it was so bad i was wondering if the mc was gone.

tom w

Jack up back of car, start car, put in gear, step on brake while observing (usually with helper) rear wheels, should stop spinning when applying brakes.
Gilly

I also doubt that 44 PSI is helping your stopping ability. If there is something wrong with the hydraulic system it is always obvious in pedal feel. If the pedal feels firm and sharp instead of soft and lazy(you should be able to make the car lurge back and forward at low speeds by idling forward, braking, idling forward again, braking, etc) then your hyrdaulic system, dare i say, is working fine.

yeah, i would bring the tires back down under 36. i usually run them a couple pounds over the recommended or at the recommended. if i were running an autocross with street tires i would run 40. never 44. 44 will probably wear the center of your tires out. it will make the car ride hard too.

good luck

tom w

cdplayer
 

Thanks guys. I borrowed a shop CD. Someone said a problem like mine may be the proportioning valve. I will keep you posted.:kid:

i dont think you have a proportioning valve.

tom w

agreed

cdplayer
 

Right you are sir. All pads same brand. Will do a flush and bleed next. According to the receipts that came with the car, the last fluid change was over two years ago. Way passed the time to do it.:)

well, it can't hurt.

tom w

What do these cars use instead of proportioning valves?

Nothing........
Gilly

well, actually they use proper design of the braking system. ie discs on both ends of the car.

i believe the usual application of the prop valve is when you have drums on the rear (to save $) and discs on the front.

so the answer nothing is correct in respect to the use of a prop valve.

tom w

cdplayer
 

Just thought I would touch base with you on the tire pressure. Did another check on the info stamped on the sidewall. Goodyear Eagle HP
205/65R15 with max. pressure 44PSI. Does seem high though.
Plan on raising rear end (of car) hehehe, this weekend to test braking
of rear wheels. Will keep you posted:kid:

Always stick to the car manufacturer's recommended inflation pressures - don't use the MAX values on the tire. On my '92 300SE, the values are inside the fuel cap door.

This is assuming that your tires are the same size as the factory specifications - MB knows what pressures will give the best combination of tire wear, ride quality and performance.

44PSI will find you in short order with 4 tires with great tread at the edges and bald in the middle - then you'll be stuck with buying new tires PLUS the cost of getting all of the fillings in your teeth re-glued after being rattled loose by those rock-hard tires.

All this being said, I don't follow the MB recommended pressures to the letter - they say 27frt and 30 rear - I find 27 makes the car too "loose" - so I run 30. So far, so good.

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

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.

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

cdplayer
 

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:kid:

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

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