The disc brakes always drag a tiny bit, which is designed to keep them free of water when riding in the rain. This is true of nearly all disc brake systems.
Is yours dragging a whole lot? If so, it may be time to replace the square (cross-section) Oring inside the moving caliper. They are 50 years old now and don't flex (nor grip) very well anymore.
The way these brakes work is: when you squeeze the lever the moving puck slides outward to grip the disc, but the O-ring around it deforms slightly rhombically to act as its 'spring. Then when you let go of the lever, the Oring returns to a square cross-section shape, pulling the puck back about 0.004" or so to clear th disc. But - if the O-ring has become too old and stiff (which all have by now) then it will just let the moving puck slide thru it instead, and the puck never retracts from the disc.
The inner puck (toward the center of the wheel) doesn't do this: it just sits in place while the moving puck pivots the whole arm and pulls the inner puck toward the back side of the disc until the moving puck retreats. But, again, if the moving puck can't retreat because the O-ring is too old, it just stays there and drags.
