On the front disks of my 85 Mercedes 300D I used Mercedes soft pads but on the rear, I did them on a Sunday and actually used Pep Boys Pads, which are probably made out of Rock (ie as hard as...).
Recently while doing my wheel bearings, i noticed the three-month old Mercedes Pads were almost worn down!!
Then I realized the mismatch in Pad hardness might have resulted in harder than normal braking pressure on my Front pas--my rears had practically no wear--- Therefore should Mercedes Soft pads only be used when changing ou front and Rear.

PS How long should soft pads last?

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