Brake disc corrosion - do they really need to be replaced?

Question

I've been told by an authorised Mercedes service engineer that all 4 discs and all 4 pads need changing on my Mercedes GLC 43.
It's just over 2 years old with 30k miles on the clock, so I am pretty surprised about this! They sent me this video of the inspection which does show some corrosion, but others seem to think this is perfectly normal:
https://mercedes.citnow.com/vtrYZKTJ3BX
They're also telling me that pads with 8mm on them need changing... which definitely doesn't sound right.
It went in because of a very occasional braking issue - brakes suddenly feel extremely heavy and it's difficult to depress the pedal. That has happened twice, spaced apart by weeks. My opinion is this can't be caused by either pads or discs, either of which would produce a consistent and less dramatic decrease in braking performance. It strikes me as more likely a hydraulics or system management issue, but they refuse to look at anything else until I replace the discs and pads.
Am I being ripped off here?
Thanks!

jwh20 · Answer

I would absolutely change those front tires and also have the brake rotors replaced.  Anytime the rotors are changed you should change the pads as well.
There may also be some air in the lines which will make the brake pedal feel "spongy" (I think that's what the mechanic meant when he said "numb") but that should be taken care of during the brake service.
The rotors look to me like the vehicle has been sitting for a long time and that has allowed corrosion to build up.  Yes, that's one side-effect of the pandemic situation, a lot of cars that were driven daily have been sitting a lot.
Could you keep driving on these tires and brakes?  Yes, obviously, but it's not safe or legal.

GdD · Answer

I would take it somewhere else for an assessment. I would agree that the tires look like they need replacement but not so with the brakes as they didn't check what they should have, or address your other issues. In any case you can back up the tire assessment by measuring using a gauge.
There is certainly corrosion there, but nothing that appears so bad as to be unsafe or warrant replacement. Having a lip is normal and the lips can be ground off if they are too high without replacing the rotors, so saying it's lipped as a reason is BS.
The reason you should replace rotors is if they are worn down, grooved or warped beyond tolerance, or so rusty that they are dangerous. They should have measured things to give you a proper assessment, this guy didn't even take the wheels off.
The grooves in at one rotor did look bad and may warrant replacement of both rotors and pads on that end, but to say they all have to go on the basis of a 30 second visual assessment is not good practice.

blacksmith37 · Answer

Corrosion had nothing to do with the disc wear; I would find a different garage rather than one that does not understand disc wear. Disc wear depends on the pad material , metal filled and ceramic causing more wear than organic . At 85,000 my rotors show no wear with organic pads , but the organics would not hold up to severe conditions like road racing.

Answered by blacksmith37 on June 15, 2021

Brake disc corrosion - do they really need to be replaced?

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