I had a similar problem with the front brakes on my 190E. I thought it was the pistons and after rebuilding them it did not go away. It ended being the master cylinder not releiving pressure in the lines.
Does the brake pedal seem unusually firm at times? Does cracking open the brake line at the master cylinder help (at least temporarily) as Larry suggests? If so, then I believe the problem oints to the master cylinder. I think you can get a new one for a little more than $100.
'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)