dave1shere,

The space in the calipers for pads is often where the burned pad material and other brake wear materials collect when the pads wear down. Then, over time, they can form a pretty tightly adherent layer of corrosion products as the carbon and metallic wear particles are not galvanically compatible with the caliper steel. This stuff gets pretty hard, and it will encroach on the space available for the new pads.

I scrape the area with a screwdriver, and use brake cleaning fluid with a soft wire brush to get the surfaces that control fit clean. I also always change brake fluid when I change the pads. The cost of a couple pints of brake fluid is negligible. Leaving old fluid in there to corrode your brakes from the inside is expensive. I also never force the old fluid up into the supply lines. I open the bleed screw when I push the caliper back in, after putting a little rubber hose over it to direct the flow to a bottle.

The pad is forced into hard contact with the disc by hydraulic fluid pressure when you step on the pedal. The pressure is relieved when the pedal is released, however there is no "sucking" on the back of the piston to draw the pad away from the disc. The mechanism that pulls the piston back is the seal ring that sits inside the piston cylinder bore groove, and contacts the piston outside diameter. As the piston moves forward, the distance it moves is very short. The seal does not slide, it sticks to the piston, and the small motion when you brake deforms the seal. When the pedal is released the energy stored in the deformed seal is what pushes the pad back away from the disc. As a result the actual pad motion from application of pressure is very slight. The amount of fluid flowing in the lines is also very slight. Lots of fluid flowing to make the pads grab the discs and then flowing back to the master cylinder to release the brake would set up unmanageable timing issues for regular braking on all 4 wheels and make ABS the way it is presently implemented impossible.

When the seal wears, for a variety of reasons not the least of which is old fluid and corrosion of critical areas, there is nothing to pull it away from the disc when the brake pedal is released. This can lead to the brake sticking. The cure is a new seal or, if the caliper or piston are visibly corroded in the sealing area, a new caliper. While frequent fluid changes will not prevent this altogether, it will successfully put it off for many years.

So, it is not unusual to have a new pad a little tight, especially if the caliper is coated with old pad dust that has corroded into a hard layer in the pad "guide channel" in the caliper. It is also not unusual to wear in the pad a little, if it is slightly too tight. I prefer to clean the caliper out thoroughly to avoid getting the pad jammed in its channel as it wears and moves toward the disc.

I also use OEM pads. I have never had to sand a pad, but I have had tough times chipping the crud out of the caliper guide channel. Other brands of pads may make this problem worse if the tolerances are not the same.

I hope this helps, 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)