You always have drag with disc brakes, but you should not hear it. Are the pads
too close to the top of the rotor, or is the rotor dished? If the rotor is dished
you may be able to hear those edges drag on new pads. If they are NEW rotors
and NEW pads, well, that is odd indeed, unless the pad is slightly over the outer
edge of the rotor, or hitting the hub of the rotor...
I have changed pads on cars for over 30 years, and never heard brake pads
dragging. If the rotor has rust spots from sitting all weekend, I hear that for
a little while then it goes away.
Common brake noises and causes aren't even included in your description,
they go as far as scraping, but that means debris caught between the pad
and the rotor.
Pistons do not really retract when you let off the brake. Pressure is released,
then the pad wears till the drag is acceptable. Rear ones cannot retract
because the plunger twists as it comes out, and you need a tool to twist
it back in. This is how I see it, if wrong, oh well... I don't see them retract
at all.
BMW has has gone as far as to put two holes in the top of their pads, one at each end, and a metal
piece, like part of a coat hanger, spans across into the holes on both pads, so when you let up, the
metal thing pushes the pads apart as far as they will go. There is not enough push to effect the plungers.
Brake shims can cause more drag than not:
What was a eye opener for me was working on a friend's car. His front tires were nearly friction free
when turning, but mine were much tighter. Ends up he didn't have brake shims installed, but I did AND
I also had CRC brake silence compound. I ditched the shims, cleaned off the CRC goo, and now mine
too are nearly friction free.
Sorry, but I never use shims either... Of the pads come with shims, I don't install
them.
You can never eliminate drag with disc brakes, just make it as light as possible.
Many newer calipers do have any kind of return mechanisms.
The pads ride in metal channels in the caliper, and rust will also prevent
any kind of returning for the pad. One cure for this issue is to grind about
.020 inch from one end, or .010 off both ends of the supporting backplane
of the brake pad. I have SEEN shops do this to deal with the rust where
the pads slide in.
Also, The caliper will rust under the copper pad clips (if there), which also binds up the pads.
The pad clips may be smooth, the the rust under them pushes them out, and the pads don't
slide freely. Grinding will still work.
I put anti-seize on the pad backplane ends, back of the pad, and in the grooves. I clean
the grooves with a wire brush first... Oh, and as I said, no shims, no brake quiet anything...
Other than the rears being twist in, I can push the plungers in with my bare hands,
thumb on each side, and press, HARD. If I can't, then I decide I may need new calipers.
Sometimes it hurts, but I can push them in, if they're not the twist in type. I was amazed
at how easy my charger plungers pushed in, front and rear... The fronts on my Jettas
I was able to push them in myself. Had to buy the tool for the rears...
