Check pad wear and replace your brake pads if they are worn more than 75 percent. Replace both right-side and left-side pads at the same time. An easy way to replace pads without opening the bleed valve to get the piston to move back is to remove the caliper, use an old brake pad and a C-clamp and tighten the clamp until it pushes the piston back into the caliper... no mess and no chance of getting air in your brake lines. Reinstall the calipers and check the brake fluid filler for contaminants and level. If the brake fluid looks dirty, it would be a good time to replace the fluid by bleeding your brake system with new fluid.