Could "rust" be the cause of squealing or dragging disc brakes? The importance of inspecting pad pins

When unpleasant squealing or dragging noises like "key" or "sha sha" persist even after releasing the brake lever or pedal, the cause may lie in the sliding parts inside the caliper. Among these, lubrication failure of the pad pins that support the pads is often overlooked. If the pins rust due to insufficient maintenance, the pads may not easily disengage from the rotor even when the brakes are released. This article explains the mechanism by which disc brake pads return, the reasons for abnormal noise, and the importance of daily maintenance.

Mechanism by which brake pads disengage from the rotor when the disc brake lever is released

The one-sided pin slide caliper is common for 125cc scooter brakes. While pad replacement can be done with the front fork still attached, it's more efficient to remove the caliper as an assembly with the caliper support if you're also performing caliper maintenance.

In hydraulic disc brakes, operating the brake lever or pedal pushes the brake fluid, which in turn pushes the caliper piston, applying pressure to the brake pads against the rotor to generate braking force. However, there is no component like the shoe spring in drum brakes that actively pulls the pads back when the lever is released. Despite this, when the brake lever is released, the pads slightly disengage from the rotor, allowing the wheel to rotate smoothly.

The key to this mechanism is the piston seal built into the caliper. The piston seal, a rubber seal with a rectangular cross-section fitted into a groove in the caliper body, deforms slightly and twists when the piston advances due to hydraulic pressure when braking.

This deformation is based on the elasticity of the rubber, and when the lever is released and hydraulic pressure is removed, it generates a force that attempts to return to its original shape. This restoring force causes the piston to retract slightly, disengaging the pads from the rotor. The amount of movement is only about a few hundredths of a millimeter, but it is sufficient to reduce dragging.

Additionally, the rotating rotor has a slight runout. This runout also helps to push the pads back. In combination with the restoring force of the seal, an appropriate clearance is maintained. If a part of the rotor bends due to a fall, a regular "shh, shh" rubbing sound may occur in sync with the tire's rotation, indicating that the gap between the pads and the rotor is uneven.

In a one-sided pin slide caliper, the caliper body slides smoothly on the slide pin, ensuring that equal force is applied to both the inner and outer pads. Proper functioning of the slide mechanism after brake release allows the pads to return to their correct position.

In other words, disc brake pads are not pulled apart by springs or other forces; rather, a very small gap is maintained by the elastic restoring force of the piston seal, the slight runout of the rotor, and the slide pin (in the case of pin slide calipers).

Brake "squeal" and "dragging" occur when pads do not disengage from the rotor

In this model, a clip is incorporated to prevent the pad pin from coming out, so it must be removed before loosening the pin. The method of preventing removal varies depending on the caliper.

After removing the clip set on the brake pad side, loosen the cap bolt type pad pin. If the pin is tightened firmly, temporarily securing the caliper to the front fork will provide stability. If rust has progressed inside the hex hole, remove the rust with a pick-up tool or similar before inserting the hex wrench. Applying loosening torque with the wrench's grip shallow due to rust risks stripping the hex hole.

As explained earlier, disc brakes are designed to maintain a slight gap between the pads and the rotor after the lever is released. However, if the pads remain in contact for any reason, it can lead to abnormal noise and friction loss.

Even if the contact pressure is not enough to significantly slow down the vehicle, friction occurs due to the high-speed rotation of the rotor while driving. As a result, the wheel's rotation becomes heavier, fuel efficiency deteriorates, and pushing the vehicle becomes more difficult.

Brake squeal is more bothersome than dragging. When vibrations occur while the pads are in contact with the rotor, these vibrations manifest as sound. In particular, if the corners of the pads, the backplate, or the caliper resonate, high-frequency sounds like "key" or "kyuru kyuru" can occur.

This is because the subtle vibrations generated by friction are amplified to become audible. Therefore, brakes do not necessarily squeal only when applied strongly. Squealing can also occur with continuous light contact.

If dragging causes the rotor temperature to rise, the surface of the pads in constant contact can overheat, alter, or the friction material can harden. Significant heat generation can also affect the brake fluid and lead to uneven pad wear.

Therefore, if the wheel's rotation is noticeably heavy, or if the rotor is abnormally hot after driving, inspection is necessary. Brake squeal is often thought of as just an unpleasant noise, but it can also be a sign of dragging or malfunction. Thus, if abnormal noise persists, it is important to check the movement of the pads and caliper.

Pad pin problems can also affect pad movement

Although completely invisible from the caliper surface, the removed pad pin is covered in rust all over. Since the brake pads slide on this pin, you can see that rust hinders movement. Not only rust, but the pads can also bite in and wear unevenly.

In this brake pad, one side engages with the pad pin and the caliper support's slide pin at two points, while the other side engages with the caliper via a protrusion and recess, with the pad pin providing positioning. The pads need to be removed not only during pad replacement but also during caliper piston manipulation.

While being careful not to let it fall out of the caliper, repeatedly squeeze the brake lever to push out the piston. The exposed parts are not only dirty but also show initial signs of rust, indicating it's time for maintenance.

After cleaning the dirt from the outer circumference of the piston with a toothbrush and neutral detergent, spray a rubber seal assembly agent, such as CCI's MR20. This is expected to reduce friction loss with the piston seal and provide rust prevention. However, applying too much can attract dirt, so wipe off any excess that drips with a cloth.

One-sided pin slide calipers are a common structure used in many commercially available vehicles. In this type, one piston presses the pad, while the caliper body itself moves on the slide pin to transmit braking force to the opposite pad. Therefore, it is crucial that the slide mechanism functions properly.

If the lubrication of the slide pins deteriorates, the resistance to caliper movement increases. This makes it difficult for the caliper to return to its original position after the brakes are released, potentially causing the pads to remain in contact with the rotor.

While slide pins are greased, prolonged use can cause the grease to dry out, or water ingress can lead to rust, significantly impairing their movement. The pad pin, a crucial part alongside the slide pin, is often overlooked. The pad pin's role is to position the brake pads and prevent them from falling out, but in many models, the brake pads slide slightly on the pin. Therefore, it is important that the pads can move without resistance when the brakes are released.

However, if rust or corrosion occurs on the pad pin, the sliding performance of the pads decreases. In vehicles that frequently run in the rain or have not been maintained for a long time, a layer of rust can form on the pin surface. This can cause the pads to catch, making it difficult for them to return to their original position after brake release.

In this condition, even if the restoring force of the piston seal is normal, the movement of the pad itself is hindered, leading to dragging and abnormal noise. In reality, it is not uncommon for dragging symptoms to be resolved by cleaning the pad pins when no abnormalities are found in the caliper or piston during dragging repairs.

While dragging is not solely caused by this, caliper piston seizure, seal deterioration, or defects can also be causes. However, slide pins and pad pins are strong points for checking. Pad pins, which are not typically treated for rust prevention, rust much more easily than slide pins and significantly affect pad operation.

Rust removal and greasing of pad pins resolve dragging and squealing!!

For shallow rust, rub with sandpaper; if the rust is deep and uneven, smooth the surface with a file. If clear step wear is visible in the contact area with the brake pad, replace the pin with a new one. At that time, consider choosing stainless steel pins from aftermarket parts manufacturers, which are effective against corrosion.

Pad grease applied to the back of brake pads and shims can be used with confidence even at high temperatures. Applying grease is important, but be careful not to over-apply, as it can lead to trouble.

The pad pin is in constant contact with the two brake pads. Although the pad movement is only a few hundredths of a millimeter, smooth movement is paramount.

Regardless of whether it's a pin slide caliper or an opposed piston caliper, the role of the pad pin is crucial. Rust or wear marks on the pad pin surface prevent the pads from moving smoothly. Clean the removed pad pin with a cleaner and remove any corrosion. Simply removing rust can restore pad sliding performance and improve dragging or abnormal noise.

Steel pad pins have surface treatments like plating, but rust can occur due to rain driving or moisture from the ground during storage. Aftermarket stainless steel pad pins are available, and replacing them can be an effective rust prevention measure.

Even with genuine pins, applying grease to the pin surface provides rust prevention. Using heat-resistant pad grease maintains lubricity even when heat is applied from the pads.

Applying grease to the pad pins is naturally effective not only for rust prevention but also for improving pad operation. When the piston retracts due to the elastic deformation of the caliper seal after releasing the brake lever, and the pad also moves smoothly along the pad pin as the pad opens slightly, it will not drag on the rotor.

However, caution is needed regarding over-application of grease. Applying a large amount can cause excess grease to flow out and adhere to the pads or rotor. Oil adhering to the friction surface reduces braking force, which is extremely dangerous. Furthermore, if pad dust adheres to over-applied grease, it can actually increase friction in the contact area with the pads, leading to poor movement.

Pad pins are parts that are always removed and reinstalled during brake pad replacement or caliper piston manipulation. It is important to make it a habit to remove rust from any rusted pins and apply an appropriate amount of grease, rather than overlooking them with the thought that "the purpose of the work is pad replacement, so the pad pins are irrelevant."