Right. All those that think its a good idea to use copper grease on your caliper sliders, please supply one piece of evidence that says this is the correct product to use. Its not. Ring round and see whats specified. Specified mind, not the hear say of your local mech or some fashionable hear say on a forum, of which there are many .
Not one manufacturer uses copper slip as a lubricant on their brakes calipers NOT ONE. Why? IT IS NOT A LUBRICANT and especially has no heat resistant lubrication properties what so ever. Read the back of the tin or tube, it makes no reference to lubrication.
I have personally seen numerous brake calipers seized solid due to this crap smeared all over the sliders which dry out and seize. Then as the pads wear and the piston pushes out to take up that wear, the sliders cant centre on the disc, you loose half your brakes power as the opposite pad does not touch the disc because the caliper cant centre on the disc, and they eventually bind on solid. Some have needed the sliders pressing out they are that seized.
To the op. Chock the relevant wheels. Jack each wheel in turn and spin by hand. They should spin freely, possibly with a light shooshing as the pad loosely brushes the disk. If not your breaks on that wheel are binding. Remove the wheel. You should be able to push the piston back in slowly by hand when the caliper is still bolted on. Push the caliper over to the inboard side (it will be stiff and move slowly with a good firm grip, and you will need the reservoir cap off to allow the fluid back up. Watch it does not over flow ) once the pistons back in the caliper sliders should move freely on the pins.
If not you need to remove your brake calipers and check the rubber boots on the sliders and piston are still sealed and not split perished ripped letting in water and corrosion, they should be seated correctly. If the sliders are seized push the rubber boot back and pray with wd40 or plus gas to free them off. Seperate the carrier from the piston and clean the brass slider pins. The correct heat resistant "grease" should be used on the sliders. Re assemble and check.
If the piston seals to caliper see corrosion on the piston inside the piston boot, it will drag the dust and or fluid seals out or damage them. This can stop the piston retreating as it should when you take your foot off the pedal.
See how you go. :-)