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My rear pads look like the middle one below:[img] 
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Are they done? I've previously had Avid pads and these wore down until being flush with the backing plate. Do the Shimano not do the same?
The brake isn't functioning as well as before, but thought it could be contaminated or glazed.
Once its gets to the spring they're past dead.
Judging from the shadow of the braking surface, there is still (some) life left in the middle one, but usually it's a case of "if you have to ask...".
I usually install new ones and put the old in the "just in case" box, which of course I will never use.
Yea, the spring is the wear indicator! Oh - how I miss Maguras magnetic pad/piston system.
Well the spring is what I'm not sure about. On Avids the spring was a wear indicator, but I'm not at the spring yet on these Shimano pads.
I did think they might be glazed, but after a quick sand they're getting shinier.... I think I'm down to the backing plate in that case!
Replacements aren't cheap but to be honest they've lasted ages