With the answers to your questions, I guess it to be the brake pads/disks as well. It sounds like the pads are not pulling away from the disk surface quick enough when the pedal is released. The previous posts seem to hit the points I would think of as well:
- rubber brake lines are swelled and constricted (common problem) and are not letting the fluid back out of the caliper.
- brake caliper pistons (or their rubber seals) are dirty or degraded and are making the piston stick in the cylinder.
- dirty/rusty pins that guide the brake pads are restricting movement.
If you're handy yourself, you could inexpensively replace the rubber brake lines, flush/refill the brake fluid, and examine the whole brake assembly (remove pads and caliper, examine the pins, piston seals, pads, disks, etc for anything unusual). Maybe it just needs a good cleaning. If you're not a DIY person, then you can have it done by your regular mechanic.
On my SL (a little older), I went about this "look and see" procedure and ended up replacing everything. New rotors, pads, rubber brake lines, rubber piston seals, and new pistons. I cleaned every associated part I could. Of course, I replaced the wheel bearings while they were accessible.
1980 450SL @ 103,000 miles
1966 230SL @ Unknown Milage
2006 C230 @ 110,000 miles (Sold)
1994 E420 @ 118,000 miles (Sold)