Matt,

This is the result of the caliper piston to caliper bore seal aging and losing its resiliency. The only thing that pulls the piston back when you release the brake pedal is this seal. It has a relatively high coefficient of friction against the piston and when the brake pedal is pressed, the hydraulic pressure created pushes the piston toward the disc a very slight distance. This seal deforms and stores a little energy. When you take your foot off the seal tries to return to its original shape. To do so it must pull the piston back slightly.

I repeat, this is the only mechanism in the system for pulling the piston back. When this seal ages, or gets worn from debris from corrosion or other souces of contamination, it slides along the piston instead of deforming and no energy is stored. The piston does not return.

I have a great deal of difficulty envisioning a failed hose acting like a perfect valve. For the hose to hold the pressure, it would have to truly be a zero leakage seal, as the volume of hydraulic oil that needs to be returned to the reservoir to reduce the pressure to its at rest pressure is next to nothing. It is this way on purpose. If a lot of fluid had to flow to apply pressure to the piston, it would result in each brake's performance being very sensitive to the flow path configuration. All brake lines, for example, would have to be precisely the same configuration or they would not activate at the same time.

So, remove the calipers from the wheel. Then push the brake pedal until the pistons pop out, and then clean the brakes until they are pristine. Examine the piston and bore, and if there is any significant scratching, corrosion or other signs of the plating coming off, the calipers are toast. The dimensions for the fit of the piston to the caliper bore are unbelievably tight and in my opinion they cannot be fixed by anyone who cannot remove the old plating and any signs of rusting, then replate and remachine/grind the parts to achieve the original dimensions. Anyone trying to do this themselves is looking for trouble with a major safety system in the car.

If the surfaces are in good condition, you can replace the rubber parts and put the pistons back in. Be very careful to keep things clean and well flushed with new brake fluid while you work. This will fix your dragging problem and likely return the original feel of the brakes. The kit of rubber parts for rebuilding is less than $25 for a set of two for most cars. I got mine at FastLane the last time I did them and it worked great.

Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)