ps2cho, no real need to measure the disc thickness. What you should do is measure the run-out with a dial indicator to ensure that disc runs straight and true when turned. Typically, front discs will last through 2 sets of front brake pads, then need to be replaced. If it's been a while since you last replaced front discs, simply replace the front discs. I always buy new discs from our local MB dealer. Be sure to replace the brake pads too, and seat them in properly so as not to warp the new discs from the jump.

As Frank says caliper pistons should retract easily with moderate force.

If you repack/replace the front wheel bearings there is a precise procedure for repacking them. Be sure to follow it to ensure long bearing life.

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Fred Hoelzle