You need to periodically pull the pads and clean the rust and accumulated crud off the parts of the caliper the pads ride on, or they will rust in place. Most common on user maintained cars because none of the books are clear on this.
My front pads had 60,000 miles on them on the Volvo, and were starting to act funny. Bought new pads and discovered that the old ones were acting funny because they were rusted in place. Had to drive them out with a hammer they were so stuck. Only worked under very heavy brake pressure, was only using the rears to stop.
My mechanic pulls the pads and cleans everything while his is doing the 30,000 mile service on cars. Also lubricates the throttle linkage, etc, etc.
Check your rotor thickness, too -- if the rotor is too thin, the pads can hit the anti-rattle spring and cock, jamming the pistions on dual piston calipers.
Get rid of the Pep Boys pads -- OEM work better and usually make no noise at all. Last about 30,000 miles in normal use, need new rotors about the third pad replacement.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!