Potentially catastrophic disc failure
 

Hello All,

The brakes on my 300TD were feeling spongy lately and the car was pulling to the right under braking. I was thinking maybe a bad master cylinder. Before I got a chance to fix it, the left front brake did a grinding noise for a few seconds and then stopped doing it. Braking had gotten worse. When I got home, I pulled off the wheel and had quite a shock. The disc part of the brake disc had separated itself from the hub part, and was seized in the caliper, which had a pad which the lining had broken off. The hub part was spinning freely, which had allowed me to nurse it home. I guess the lining had broken before this, as that side of the disc was covered with road salt crust.

Has anyone else heard of this happening to someone?

Chinese brake rotors?? I did see this happen on a Japanese car once a long time ago. When something feels wrong on a critcal system it pays not to take it to failure...Have you ever heard of the term "Jesus nut" used by airplane mechanics??

Nope, never heard of it.

I'm usually not one to push back critical maintenance, but this one happened while on a trip, so I chose to "keep on truckin". There is a good time and a bad time for partial brake failure. I guess I was lucky that it happened at a good time for me, on a quiet country road and not on a busy highway.

Car should be back on the road tonite, the auto parts store has what I need in stock.

Are you replacing a caliper? Sounds like one of the pistons is frozen so the force was coming from one side.

A Jesus nut is the nut that secures the propeller on a airplane. If that nut fails in flight guess who you we be seeing next? Sort of like failed brakes on a car!!

I have seen this a few times on heavy winter use vehicles. The road salt just ate the rotors away until the disk separated from the hub or the bridge sections of a ventilated rotor would corrode away and the two friction surfaces would separate. Not pretty at highway speeds. Never seen it in a MB but since you are in Montreal I would say you have your fair share of road salt. Replace what needs to be replaced and rinse the brakes and suspension thoroughly whenever you can. RT

When I bought my 300TD I was shocked to find it had non vented front brakes. My gosh my 1971 300SEL has nice vented front rotors. I would have thought they would have been standard across the line by 1980. I guess the bean counters made this decision!!!

So the disc came apart from the hub..... what happened did it just break around the bolts or did they come out?

More likely used by helicopter mechanics than airplane mechanics.

Maybe but the guy to told me this was an Airforce mechanic who never came near helicopters...

Seen it before on a 70's gasser benz...front rotor did just that...was about to seperate when Les Schwab found it (a friend of dad's car). It was cracked almost the whole way around (had about 30 degrees left to snap loose) They said they wouldn't let him drive it home with that problem (he knows where to get decent prices on german parts...not Les Schwab's screwing).

I wanted that car...more leg room than I could ever ask for...it was one of the old body styles...

broken rotors
 

Yes really not all that uncommon. It raised some of the same questions as yours in my mind. Separation seems to occur or start in area between disk and high hat hub of rotor. Where the metal is thinnest. Seems to take a chunk out of the hub of the rotor sometimes as well. Probably about time of total separation. Mercedes wants less than 5 thousand runout on a good disk so borrow a dial gauge and stand if you do not own one when putting on the new replacement rotor and check it. Also make sure your front wheel bearing is set up properly. Mercedes proceedure for that is very easy as well to do. With under torque front wheel bearing will probably contribute to actual rotor wobble or runout. Certainly can't help. Clean area of rotor hub contact well to help achieve under 4-5 thousand wobble as well. (did not look up spec but pretty certain) Imagine failures are partially stress related as that hub is pretty deep. On the incidents of mine found calipers to be in good shape. Others may vary I imagine. :sun_smile

The disc part of the rotor separated all around the seam where the disc part meets with the hub part. You could see that about 1/16 in. of metal was all that was holding the disc part to the hub part. The hub was still spinning freely, with the disc just sitting still in the caliper.

Road salt is the likely culprit. All cars are crusty white up here these days. You wash your car and the next day it's crusty again. You just can't win. I try to not use the diesel for my short winter trips in order not to expose it too much to salt, I have a 89 accord beater for that. But when I need to do road trips, I always take the TD so I can burn WVO.