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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.
 

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Often overlooked are the Rr brake shoes on motorcycles with a drum brake. The adhesive lining on old Br shoes can get brittle & fail, so a lining can separate from a shoe then get jammed between the shoe & drum, locking the rear wheel. It happened to me, so it does happen.

As for disk brakes, one of the first things I refurbished on my old KZ650 was the front brake. The master cylinder upon inspection at the time was fine, so I didn't do anything to it, but I replaced the old rubber lines w new ss braided. Then I refurbished the master cylinder almost 2 years ago. But the piston started leaking last fall, so I installed a Fr brake mc from a Honda CBR500.

 

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Folks not replacing brake fluid regularly is huge. I've ridden with at least 3 different people who "lost" rear brakes and had severely overheated components. There no reason not to do it with all the choices of pumps, speedbleeders, etc. I have speedbleeders on all our vehicles.
 
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