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Old 08-22-2003, 12:35 AM
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Join Date: Aug 2003
Location: Franklin, TN just south of Nashville
Posts: 1
Unhappy My first Mercedes...Brake question...

Hello, I have owned and worked on dozens of cars and this is my first MB. 1991 420SEL (126/116?). It needs new brakes. I've changed out discs, drums, and pads several times on other cars. Will this be any different or more complicated? Should the sensors be replaced automatically too? Any advise would be greatly appreciated.
Thanks, Jeff
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Old 08-22-2003, 02:07 AM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,123
I don't know what other cars you've worked on but MBs are different from other cars I've worked on. The front hub has to come off the knuckle so you can get the rotor off the hub. You'll need new rear hub seals for sure and you might as well inspect and repack the wheel bearings. The most difficult part is measurig the proper amount of grease. It's all in the archives. There's also the issue of setting wheel bearing free play by feel or by dial gauge. Many do it by feel.

The rear is easier. The only real complication is when the park brake shoes wear a groove in the drum section of the rear rotor.

The pad thickness sensors should be replaced. The ABS sensors... I don't. I don't know if anyone does.

I wouldn't do a brake job without a proper manual.

95 S420
91 300SE
87 300SDL
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Old 08-22-2003, 07:20 AM
Posts: n/a
When you say you need brakes, that is a broad statement. If you only need pads you will find this to be the easiest brake job you ever did. If, on the other hand, you need rotors then sixto's advice will be helpful.

For pads simply remove the wheel, suck some fluid from the rear reservoir, drive out two pins that retain a leaf type spring holder, then pull out and replace the pads ONE AT A TIME. Pull one pad, pry or press the piston back into the bore and replace that pad, then pull the other pad and pry or press THAT piston back into the bore and insert that pad. If you pull both pads and then pry or push a piston back into its bore, it can force the opposite piston out past the seal lip and require caliper rebuild.

If the sensors are in good shape you shouldn't need to replace them, but if the dash light is on, then at least one has worn through and really should be replaced. The sensors are less than two dollars each and I make sure I have a set on hand before I tear things apart. I then replace the one(s) that are worn through. They simply pull out of a hole in the pad and can be removed from their connector on the other end with a pair of pliers by twisting and pulling. Make sure the new ones are firmly in place on the connector end.

It is also not a bad idea to open the bleed screw on the caliper you are working on to prevent the dirty fluid from being forced around the piston seals. If the system has been kept clean by thorough annual flushing, this step should not be necessary.

With the new pads in place, flush and bleed the entire system THOROUGHLY. If you will thoroughly flush the brake system annually you may very well drive the car for years and years without doing any hydraulic repair, only friction components.

If you are using the "two man" bleeding method, you may very well have to do it with the engine running to provide brake boost. Some of these cars just can't be "two man" bled without doing this.

Good luck and let us know how the job turns out.

Have a great day,
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Old 08-22-2003, 08:04 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Rotor thickness is critical on a Benz. MB uses thin rotors to reduce unsprung weight, and this means that there isn't anywhere near as much "meat" there as there is on most other cars. They don't warp, but they do need to be replaced every other or every third pad replacement.

There is a minimum thickness stamped into all OEM rotors on the circumfrence, but this may be hard to read and won't be present on off brands anyway. The quick and easy way to tell if you need new rotors is to check for a disctinct lip on the outer edge, where the pads down quite cover, and at the center beyond the pad area. If there is a lip, or the friction area is worn down the the base at the center, don't bother to measure, replace. They are very inexpensive.

You will discover if you try that they will be too thin if you turn them.

Front rotors on models before the W201/W124 chassis are bolted to the hub from the back, so the hub must come off to repalce. Later models are held on by the lug bolts.

Clean the bad slots in the calipers carefully, crud here can cause squeals and sticking brakes. Use on OEM pads, and put anti-sieze on the back of the pad where the piston contacts the pad and the sides of the backing plate were it contacts the caliper to prevent squealing.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 08-22-2003, 09:55 PM
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Join Date: Mar 2002
Location: Great White North!!!!
Posts: 400
if you want the best 126 site:

try this guy:Thomaspin
do a search for any of his replies and then follow it to his site.he covers everything and does a great job of explaining
'95 s500 (bought for wife but can't bear to share!!!) 125kms
'92 legend 180kms
'88 tbirdturbo(fantastic car-only regular maint.)120kms
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'86 tbirdturbo(gone)
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Old 08-22-2003, 10:11 PM
Posts: n/a
Re: if you want the best 126 site:

Originally posted by daddiojiggy
try this guy:Thomaspin
do a search for any of his replies and then follow it to his site.he covers everything and does a great job of explaining
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Old 08-23-2003, 01:09 PM
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097
Here is a helpful and seemingly proven easy "guide" for disk thicknesses:

If your brake light is on and you have a lip whose thickness is no more than that a "Dime" - I might used them again.
"Pennys" - its about time to change them
"Nickel" - Regardless brake light or no hurry up and change em'

Here are the official thicknesses of the US coins:

Penny 1.57 mm
Nickel 1.98 mm
Dime 1.35 mm

Source: Penny Dime Nickel

For pads I am totally tired of brake dust. I have been using PBR Delux pads. Work very well and almost no brake dust.

'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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