I found my brakes were running hotter than I though they should and went through my pads a bit quick on my 190E 2.6. I thought it was the pads not releasing. When I replaced the pads, I found it quite difficult to get the piston back into the caliper. I got caliper rebuild kits, cleaned the bore and pistons and replaced the o-rings and boots.
The next set of pads were gone in two weeks. I found my problem was the master cylinder. The master was not allowing the fluid back into the cylinder. I had to actually get out of the car and release the pressure in the brake line by cracking open the brake fluid line on the side of the master cylinder. It would come squirting out and all would be fine for a little while. I would have to do this 4-5 times each way from work. Once I had to release the pressure twice in less than 100 yds of driving.
I finally got the new master cylinder and all is well. Of course, I never had to replace the rear pads the whole time. It shows you how much braking is done back there.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)