This ‘ible details how to go about fixing your motorcycle brake caliper when the brake lever starts getting uncomfortably close to the hand grip with resulting loss of braking power. If you pump quickly once or twice the brakes work again, but after you release the lever for a short while, the next pull on the lever will again result in the brake lever sinking almost all the way back to the hand grip. This behavior feels EXACTLY like air in the system. If air was in the system, then bleeding the hydraulics would fix the problem. But, for this problem, you can bleed the brakes until you are blue in the face and you won't have fixed anything.
While this 'ible will detail the repair process for the front brakes, the procedure for the rear brake caliper is the same. The steps also show the removal and re-installation of the brake pads, so you can use this guide for changing your brake pads as well. Think of this as a twofer!
Before you do anything, please be aware that proper functioning brakes are critical to your safety as well as the safety of other road users. Brake failure can result in death. If you are unsure of your mechanical abilities, please do not attempt this repair.
I developed this method from a vague hint on a motorcycle forum as to what the problem might be. I hope you find it useful.
Step 1: How the brakes are supposed to work
The problem occurs gradually over weeks of riding which gives you plenty of time to correct the fault. Once the problem has started appearing and the bike is not ridden for a week or 2, the failure can be more sudden. Fortunately you will feel this during your pre-ride check after the bike has been garaged for a while (and I’m assuming you ALWAYS check your bike before riding… right?!!)
The brake system has a ratcheting action. Every time you apply the brakes, the pistons press the brake pad against the rotor creating friction which slows the bike down and also results in loss of material on the brake pad itself. This wearing process occurs slowly or else we would be changing pads daily instead of after thousands of miles. When the lever is released, the pressure is released and a set of springs and a little bit of vibration and bumping from the rotor cause the brake pads to push the pistons backward slightly to remove the friction from the rotor. As the pads get thinner from repeated brake application, the pistons slowly move out of the caliper over time so that the brake lever engages more or less at the same distance from the hand grip. This is the so-called ratcheting action of the hydraulic brake system. As this occurs, the level of hydraulic fluid in the brake reservoir will be dropping so maintenance top-ups are required.
Over time, dirt and brake dust coat the pistons in the brake caliper. Some of this dirt may work itself into the piston seal when the pistons retract a bit after brake application. Sometimes, instead of the piston sliding forward through the seal when the brakes are applied, the piston gets stuck to the seal and rolls the seal forward when the brake lever is pulled. When the brake lever is released, the seal restores its normal shape causing the piston to be pulled backward. The ratcheting action is lost. As the brakes wear, the piston gets further and further from the friction point. The result is that the brake lever activation point moves closer and closer to the handgrip.
The speed at which the piston retracts back into the caliper due to elastic action of the seal is quite slow so 2 quick pumps on the brake lever generally result in the pistons making contact with the pads, and the lever works at the correct distance from the handgrip. The reason pumping works is because the first pump moves the piston outward as much as the hydraulic fluid pushed into the system from the hand lever allows, and the second pump forces additional hydraulic fluid in behind the piston which moves it out to the pad and generates enough friction to stop the motorcycle.
Crikey, a lot of words to describe this problem. So on with the fix.