Been trying to answer this for 3 days. Computer problems.
The original seals in our calipers were a cup type seal. They seal well when pressure is applied inside, but can leak easily when pressure is reversed. The advantage is a tighter seal as the cup section is pressed against the caliper bore when the brake pressure is applied.
O-ring seals do not do that. The maintain equal piston to bore pressure (side to side) no matter how much pressure is applied.
While you don't get the additional sealing force with the 0-rings, you don't really need it. Overall they are quite an improvement. The cup seals when relaxed for a long period of time can actually allow air to enter the caliper. This is why we often need to bleed our brakes after a winter nap. O-ring seals won't do that. If you need to bleed them it's for a different reason.
Neither seal has any tendency to retract the pistion. New square cut seals will flex sideways when applied, and pull back a bit when released. C3s don't have square cut. In fact they have a spring behind each brake pistion to help keep the pads out by the rotors. Without the springs, the pistons will retract a bit due to road vibration, rotor flex on corners, etc. After a run down the highway or a few wicked corners, the brake pedal can become very low. You may need to pump the pedal one time, or several times to push the pistons back out to the pads and rotors. That usually creates a real OH CRAP moment, then the car stops.
As Joel stated, the do get pushed back just a tad based on rotor warpage, and movements, but stay just brushing the rotors.
The brake dust is all about the pads. Some do, some dont. I have organic similar to OE and do not have a dust problem. Ceramics usually don't dust, but that depends on the brand. Semi Metallic will almost always give you dust. Pads are not created equally. All similar pads are not a pure mix, but a complex blend of many materials. Ceramics and not all ceramic. Every style of pad is a blend of many things. This being the case, a blanket statement about which type may not be valid. Name brands are a lot more specfic.
So which type of organic pad do I have on my car? Please don't cringe, They are from AutoZone. And yes, I'm very pleased with their performance and lack of dust. Are they great for everything, no. But I don't go racing on track days, just drive on regular roads and highways. (Okay sometimes you would think I'm on a track, but we won't talk about that).
