Some information to aid diagnosis of brake problems. As a retired professional mechanic and auto, motorcycle, aircraft enthusiast I follow multiple forums. One issue that comes up periodically is dragging brakes. The answers always suggest problems with the caliper which may well be the problem. However there is one important factor that is consistently overlooked. That is the master cylinder compensation port. I believe that most think of the brake system as a closed hydraulic system. It is actually open until the brake is applied. If the system were closed full time the fluid would have no where to go as it expanded from heat. Also as the pads wear there would be no way to introduce fluid to compensate for the increased caliper volume due to thinner pads. There is a very small hole just in front of the retracted master cylinder piston which allows heated fluid to be pushed up into the reservoir and also allows fluid to flow out of the reservoir to compensate for pad wear. As the brake is applied the piston covers this port and closes the system allowing pressure to build. One of the most overlooked and neglected systems on any motor vehicle is the brakes. People go years and sometimes the life of a vehicle and never change the brake fluid. Brake fluid is hygroscopic and over time enough water is absorbed to corrode everything in the system. This crud can accumulate on the bottom of the master cylinder. If the port becomes clogged the system is now closed full time and the brakes will not release consistently or properly. The more the brake drags the more heat is generated and the greater the expansion of the fluid.