Front Brake pad outside wear only?
 

I changed the front pads on my 1993 500E and noticed only outside wear down to the sensors and the inside pads were 80%. What is going on?

This is a guess, because I'm not familiar with the 93 500E, but if it has 2 pistons, one on each side of the caliper, the inner one is probably siezed in the bore. If it has only 1 piston (or pistons) on 1 side of the caliper, it should 'float' (move in and out) when the brakes are applied so that equalizes the pressure on both sides of the rotor. In this case, it sounds like the slides (the cylinders running over the caliper mounting bolts) are siezed.

There are two pistons on each side of the caliper. I find it suspicious that both front brakes the inner pistons would are seized. I could easily move the pitons when I put the new pads in and before I rebuild them I'm wondering if it could be something else?

Are they 'floating' calipers, with 4 pistons, 2 on each side then? ie., Can you move the entire caliper easily from side to side on pins/slides? If they are, and you can't, then that would be a possible too. If not, it could be that the hydraulics are at fault, where the outside pistons are applying more pressure than the inside ones..... I'll exit the thread at this point so someone with actual knowledge of your set-up can comment. Good luck

That car has 4 piston fixed calipers, so there are no slides. Did the pistons slide back in easily on both sides of the caliper after the pad change? Was there more resistance on any of the pistons? The first possibility would be a sticking piston.

Pistons moved freely, although getting the inner pads out were PIA. I cleaned the top and bottom surfaces a little. Anyone lubricate the edges to slide in better?

I have to admit I'm stumped on this one, and I agree with your logic about both sides being the same. The only guess is a mfg flaw in the original calipers like internal bores being incorrect size. If you don't get an answer in a couple of days, I'll post it in the techs only section. I'm curious.

Peter

I was very interested when I read your post. I have a 400e 93 and I too have experienced uneven wear on front brake pads. I recently replaced the rotors and pads after the "brake wear light warning indicator" came on. I found it very unusual to see the pads on one side of the rotor with most of the wear. I bleed the brake lines and I cleaned the caliber and surrounding parts well with break cleaner and used break lubircant on the pistons thinking it might be a siezed caliber piston. My 400e brakes are the same as the 500e 93 model - dual piston, fixed caliber.

I only have less than 1,000 miles on the new rotors and pads so I'm unable to say if the recent work has helped address the uneven wear on the new brake pads.