The braking has felt fine. And since I own and drive two identical cars, I'm usually pretty sensitive to even minor changes/differences.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Well, if that rotor was not getting contact (or very little) and since the front brakes actually do most of the stopping of the car - it may have not been noticeable. Now, under full braking, I think you might sense or notice a difference between full-contact, 4-wheel disc braking, and not. It's good that you noticed something to look into. This thread is very helpful, since I think I ought to do more inspections on my own 4-wheel disc brakes/rotors vehicles.
I'll be interested in learning the source/reason is, for what you found on your car.

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'06 E320 CDI
'17 Corvette Stingray Vert

Kris, just out of curiosity (and my inherent cheapness), why in your opinion is this worth the extra money? If the first set of rubber hoses lasted 21 years and 242,000 miles, why not get new rubber hoses for $5?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

The rust band can occur even with a properly functioning caliper.

What happens is that the edge of the rotor rusts a little, turns into sand paper and wears the pad away at a faster rate than smooth steel. Now that the pad no longer wipes the surface, the rust continues to grow wearing more of the pad. With this much rust, it is a definite safety inspection fail.

Check the operation of the caliper pistons as part of a proper brake replacement.

With this kind of rust, driving the car normally won't show any odd behavior, only hard braking will net early front brake lockup, you may also get pull if the rust is significantly different from side to side.

The second type of rotor rust would be scattered islands of rust sometimes in the shape of the pad outline. This is usually caused by poor rotor quality but can occur on good rotors when metallic pads are used.

That's interesting. If in fact the car is driven virtually every day, is it a safe assumption that the only way for that rust pattern to develop is a malfunctioning caliper?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

yup.

if you flush the lines REALLY well, FULLY removing all traces of old fluids, you might get the piston to release, but I doubt it.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Yes, even on a daily driver with a working caliper, the rust ring will grow. Remember, once the surface rusts and gets rough, it sands the pad down faster than the pad wears on a smooth surface preventing it from rubbing the rust off. Once the rust progresses more than 1/2 way, the pad tilts and makes full contact but by that time the rust is so well established it won't wear off.

If the caliper piston on a opposing piston caliper was stuck, the rust would extend the entire width of the rotor surface on the stuck piston side.

... nah. I've put rotors on that are totally rust caked, and with properly operating caliper, the rust is polished off in about 40 miles of driving. if he changes the caliper, the rotor will look shiny and new in a few days of normal driving.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Just pruchased a rear rotor for $20 something. At that price sort of hard to reuse something that isn't 100% known good to me.

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You can't always get what you want, but if you try sometime you just might find you get what you need.

No, it won't clean up when driving.

If you look closely, the rust is burnished and the inner band is shiny showing the OUTER caliper piston is working. Now, it is unknown if the INNER caliper piston is working.

The rotor rust is very deep and won't just wipe off. Using your logic it should clean up to a bright shiny surface with a couple of rubs with 80 grit sand paper. Want to have the car owner try it and post the results?

Just for the record, I'm not a DIY / Backyard mechanic. . . .

Here in the coldest darkest wettest parts of Europe (where it seems that spring ain't ever gonna arrive this year) you get this kind of result...



...from road salt and not much use (whether the calipers are working correctly or not) particularly on rear discs that aren't used to the same degree as the front brakes.

Once the discs are as badly corroded as this you need to change them.

Before ripping into a caliper I'd check to make sure if it works.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I have seen the kind of thing above (e.g picture from Stretch) on some GM rear brakes, the metal actually rusts somehow underneath the wear surface and becomes impressively rusty and eats pad.

But I have had brakes that looked like came from underneath the sea for 20 years on some other cars, and just started driving and looked new after 15 miles or so. I dunno what is the difference between them.

I think "shape of pad" is usually pad material that has stuck to the rotor. My CLK has this issue, the rear rotors have pad material stuck on them and when you brake, the pedal vibrates. I am not sure what to do, maybe will take a sanding disc to the rotor... new rotor is cheap but I am lazy and brakes are decidedly unsexy.