Inner brake pad much more wear then outside pad
 

I've recently finished doing the rear parking brake shoes, and noticed that on the rear left brakes, the inner pad is almost completely worn away, while the outer still has about 50% to go. Both pads on the right side also seem to have left about 50%. Both rear discs are evenly worn and show about the same texture on both sides.

What kind of problems can produce such an increased wear in the left inner rear pad? The car feels just fine and does not veer to any side at all while stepping on the brakes. Wet or dry pavement, it stays perfectly straight on hard braking.

Indicates a sticking caliper piston...usually rust/dirt under the boot..

Also another reason it is so important to change fluid every couple of years.
..or at least bleed the calipers off until the crap comes out and the fluid runs clear...that is where all contaniments wind up ...right in the caliper piston cavity.

Which piston would be sticking, the one on the side where the pad is almost gone? And, should I order the caliper rebuild kit, just in case? Thanks.

I did change the brake fluid about 2.5~3 years ago, maybe it's time for another change.

Yes.

What happens is the dirt boot leaks and rust gets on the pistons. The piston is easily moved easily when you push on the brake pedal b/c the master cylinder has such high pressure reacting on the piston in the caliper, but once you release the pedal, the piston is supposed to relax the pressure on the pad , but there is just residual force doing that and if the piston is gunked up, it sticks . The results is there is always pressure on that pad and it then wears faster than the other side.
Whenever doing pads , it is a good practice to push the piston in/out a few times to self clean them..
But a kit is the correct remedy..you will plainly see the problem when you take the piston out.
A neat tool that I use all the time for brake checking is an Infared thermometer gun. You can take wheel rim readings after a good run and a sticking calpier will show up as a hot rim.. You can also find them just by feeling with you hand. A sticking caliper or restricted brake hose causes undue pad pressure and wheel drag..... the results are heat generation on that wheel/rim/caliper.......restricted hoses are always mis-diagnosed as bad calipers, but a simple open bleeder test will differentiate the two.

Once the piston is out, as you mention, should I clean it? and if so with what? I have a soft bronze bristle brush, would it be ok to use it?

I've added the rebuild kit to my shopping list, although according to the catalog picture it only seems to include a pair of circular rubber seals, not the dust boots. Or are those the dust boots?

http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=2E204PMU62E204Q3OG&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1986&product=N1050-24237&application=000040129

Those are the boots..the piston seals are just ring seals and they are sitting on top of the boots in the picture.

Clean the pistons with anything that will not scratch them..they are unpolished chrome finish.....sometimes they are pitted to the point of junk.

Caliper repair kit and rear pads are on the way. Will report back once the job is completed. Thanks.