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  #1  
Old 02-18-2004, 12:47 PM
goldstone's Avatar
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Join Date: Oct 2002
Location: Dutchess County, New York
Posts: 156
Asymmetrical brakepad wear on rear dual caliper (W126)

Hi All!

Finally got around to doing my '89 420SEL's brakes yesterday evening. I noted with surprise that, of the 4 rear pads, one exhibited significantly more wear than the others.

Old pads were Pagid, calipers are ATE. Rotor on the questionable wheel was okay--no obvious uneven erosion (still about 50% life remaining).

I've noticed no irregular symptoms (i.e uneven tire wear, pulling to one side, poor braking), so I suspect it's just a fluke. On the other hand, it is the brakes we're talking about here, so I figure it's a question worth asking...

Anyone have any ideas about what would cause this sort of asymmetrical pad wear within a 126's dual caliper?

Thanks!

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Eric

1991 560SEL-Euro (185,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)

Last edited by goldstone; 02-18-2004 at 02:00 PM.
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  #2  
Old 02-18-2004, 02:20 PM
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Join Date: Sep 2002
Posts: 150
the non worn piston is a little stuck.

remove pads

depress brake pedal several times whilst watching pistons come out

clean with brake fluid and toothbrush.

push back in

test.

if this fails, you need a seal / piston kit
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  #3  
Old 02-18-2004, 02:44 PM
goldstone's Avatar
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Join Date: Oct 2002
Location: Dutchess County, New York
Posts: 156
Semi-stuck piston makes a lot of sense...

As a matter of form, I always scrub out all of the old brake dust and paste that tends to gunk-up these calipers--though I've always used canned brake parts cleaner vs. brake fluid to do the job. (I also use a piston-spreading tool to evenly compress the caliper pistons, which greatly eases insertion and removal of pads.)

Hopefully, between the manual spreading of the pistons and the thorough cleaning I gave the caliper, any stickiness that may have indeed existed has now been corrected.

I'll recheck the caliper/pads next time I have the car up on stands.

Thanks for the good suggestion!
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Eric

1991 560SEL-Euro (185,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)
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  #4  
Old 02-18-2004, 03:29 PM
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Location: Evansville, Indiana
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Eric:

probably time for a caliper rebuild (by which I mean replace the piston seal and dust boot).

One piston is sticking, so that it doesn't exert as much force on the rotor, OR the other three are dragging and not retracting. One stuck is more likely, even more likely if someone ran the rotors too thin and the backing plate hit the anti-rattle spring.

Rebuild on these calipers is easy -- remove caliper and drain brake fluid. Stand caliper in a vise, pry off the dust boot and remove from piston, then use two same prybars (from Autozone, etc) to hook the lip where the dust boot fits on the piston and lift piston out of caliper. Note the orientation of the piston. Your's will probably have a heat sheild -- the piston sticks through it part way round. You must put the piston back in the exact original orientation, so look closely.

Take a "twisted" pick (the funny one in the four pick set from AutoZone, etc) and pull the piston seal out. Spray out bore and caliper body with brake parts cleaner, then wash piston with brake parts cleaner. Inspect piston and bore for scratches, gouges, or galling -- usually none -- then the part of the bore above the seal for dirt, corrosion, or rust. If that part of the bore is dirty enough that the piston won't drop back in without intereference, you can clean it wil brake parts cleaner and some scotchbrite -- DON'T use scotchbrite or ANY other abrasive anywhere else!

Flush completely and wipe out bore, then coat new seal with brake fluid and install. You will probably have to use the pick to roll the last bit in, make sure it's not twisted or the piston will cut it going back in.


Coat piston with brake fluid and insert in bore. Double check the orientation, as you cannot rotate it without the special too. Place a short board through the caliper across the top of the piston, and with the caliper on the floor, push the piston back down in the bore. It will be VERY stiff until you get it through the seal. Alternatively, you can start it with the board, then use a C-clamp to press it down. It helps to twist back and forth a tiny amount to get the piston over the seal. The piston should "bounce" back some when you release pressure -- if it doesn't, you 've rolled the seal, remove piston and inspect. May take a couple tries to get it in, don't force it.

Install dust boot by snapping over the piston, then use a pair of channel locks to push it down all round -- you may nave to use a pair of vise grips to hold one side to start it on.

Press the heat shields in after you are SURE the piston is in the correct posistion -- I don't think you can get them off intact if it's not right!

Peter
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  #5  
Old 02-18-2004, 04:20 PM
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Join Date: Oct 2002
Location: Dutchess County, New York
Posts: 156
PSFRED,

Thanks for the scholarly and detailed set of rebuild instructions; REALLY nice of you to take the time.

Two basic questions, one technical the other monetary:

1) TECHNICAL: Only one pad (of the four) showed disproportionately greater wear; the other three were about the same. It seems to me that if a single piston were sticking in the retracted position, it would show LESS wear than the other three. Conversely, if an errant piston were stuck in the extended position (which would indeed result in extra wear), wouldn't it be noticable when driving? What's more, in either case, wouldn't one find obvious uneven disk wear? (I've owned this vehicle since it was only 15,000 miles old, and I assure you the rotors have never been run too thin )

2) MONETARY: Phil sells rebuilt calipers for around $75--brand new ones for just twice that. Given the tools and effort involved in the rebuild you generously documented, do you think I might not be better served by simply replacing the part instead--assuming this trouble persists?

Regards!
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1991 560SEL-Euro (185,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)
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  #6  
Old 02-18-2004, 07:54 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
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Eric:

Unless a rear brake is dragging enough to make stopping jerky, you will only notice it by reduced milage or loss of power, as both rear wheels will drag (through the differential).

It is VERY much more likely you have one sticking out that only one retracting properly! Usually the result of an old seal no longer flexing (the seal is a large square "0-ring" about 1/8" thick in a groove -- applying the brakes flexes the seal only, and the piston retracts from the resiliency (sp) of the rubber. As the pad wears, the piston slips a bit). The pad will drag more when it's not pulled back by the seal -- the runout in the rotor will push it back some, but it will wear the pad more. A bad dust boot allows dust and grit into the upper part of the bore, above the seal, and this will lock the piston in the applied position. You will get drag on that pad only until the rotor and pad heat up some, forcing the piston back. Once it cools off, it won't drag again until you apply the brakes.

Rebuild kits are about $20 each on fastlane, takes about an hour to do each caliper if nothing is really stuck.

If you don't want to mess about with the rebuilds, just send yours back as cores and pop the new or rebuilt ones on and be done with it -- it's a matter of what your personal time is worth.

Most shops are very reluctant to do even swap-the-seals brake rebuilds, too much liability, and many won't put rebuilt calipers on because they have too much trouble with them.

Peter

__________________
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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