If brakes leak fluid, the results can be deadly. For this reason, it is imperative that brakes be 100 percent leak free and functioning at all times. Wheel cylinders, found inside drum brakes, are very precise devices made with tightly fitting parts designed to hold fluid. When you press on the brake pedal, it sends fluid down to the wheel cylinders, pushing internal pistons out which activates the brakes. When a wheel cylinder leaks, something is definitely wrong. Understanding the potential causes of leaky wheel cylinders is the first step in correcting a potentially dangerous situation.
Internal corrosion is the chief culprit of leaky wheel cylinders. Brake fluid, by nature, is hygroscopic--meaning that it absorbs water. If the fluid is not changed for several years, the moisture in the fluid rises to considerable levels. The moisture, in turn, contributes to internal rusting, which eats away at the cylinder bore, leaving holes. These holes allow fluid to leak past the pistons and out to the environment.
When brakes are bled after work is done, the bleeder nipples must be closed all the way and tightened down securely according to the manufacturer's recommended torque setting. Bleeding is a process of removing all air from the system. If the person bleeding the brakes did not put the correct torque setting on the bleeder nipples, they will vibrate loose over time. When this happens, fluid leaks out from the bleeder area. Suspect loosened bleeder nipples if you had the wheel cylinders replaced but within the year the same wheel cylinder starts to leak again.
Worn Piston Seals
Piston seals will eventually wear out due to age. The seals are made out of a compound of rubber, and over time, they become brittle. When this happens, they can crack and allow fluid to leak past the pistons.
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