Brake Master Piston "Rotation" Dramatically Changes Lever Feel!
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There have been instances where, immediately after replacing the brake fluid, a strange feeling occurred, and before I knew it, fluid was leaking... I've also experienced changes in the master cylinder's operability after a fluid change. Here, we will report on "polishing the inner parts" that we recommend trying when changing brake fluid.
Even Small Touches Yield Great Results = Tangible Performance Improvements
The inner piston of the brake master cylinder is often made of aluminum, and wear can occur in the sliding parts due to poor lubrication. Here, we will polish the part that rubs against the inner wall of the master cylinder to improve its operability. The cup seal was masked with paper tape.
Holding the piston body with your fingertips, including the masked paper tape area, polish the outer circumference of the piston body that rubs against the cylinder's inner wall with 1200-grit waterproof sandpaper. Chamfering the edges during polishing appears to be effective. Be careful not to over-polish.
After polishing with waterproof sandpaper, apply metal compound to a fine-grit cloth or microfiber cloth and polish the outer circumference. Polishing reduces resistance and reliably improves operability. New parts may have scratches or burrs characteristic of die-cast aluminum, but achieving this level of cleanliness is satisfying.
Aiming for Improved Operability by Polishing Metal-to-Metal Contact Surfaces
Polish the push part of the brake piston and the brake lever using waterproof sandpaper to smooth out scratches, steps, and uneven wear. This polishing work dramatically changes the brake lever's feel. Uneven wear in this area causes a subtle catching sensation during lever operation.
Although there was no uneven wear on the push part of the brake master piston, there were scratches from the tip of the pickup tool when it was pushed during disassembly. This time, we decided to remove those scratches by polishing. First, polish with waterproof sandpaper, then proceed to the finishing polish.
For the initial finishing polish, an oil stone for rough polishing was used. Polish the end face while applying oil. Oil stones also come in different grit sizes. If there are steps, it is important to work through the steps sequentially to restore flatness.
Polishing with Metal Polish is Effective!! Reduced Resistance Leads to Improved Operability
Metal polish chemicals, which are highly effective for polishing aluminum, were used. By polishing extensively with the oil stone, a smooth finish was achieved. Now, we will proceed to the final polishing. We look forward to the feel of the lever operation.
Similar to waterproof sandpaper and oil stones, lay a fine-grit cloth or microfiber cloth on a flat board, apply metal compound, and polish the piston end face by pressing it against the cloth. Also, check the condition of the push part on the brake lever side.
Once the polishing is complete, remove the masking tape and thoroughly clean any dirt from the cup seal and its surroundings with parts cleaner. It is best to polish when the main body and seal are separate parts. Even with new parts, this procedure can yield a noticeable difference.
The inner wall of the master cylinder was polished using a nylon pipe brush of appropriate size. If a suitably sized flex-hone were available, it would polish it to a shine in no time. When assembling, applying rubber grease to the outer circumference of the cup seal dramatically improves the operability between the seal lip and the metal cylinder. However, applying too much rubber grease is not advisable.
- Point 1: If you feel a chattering sensation when firmly squeezing the lever of a hydraulic disc brake, polishing the master piston is effective.
- Point 2: It is important to polish, not to shave. Continuing to ride with chattering can lead to uneven wear.
- Point 3: Before disassembly and polishing, cleaning the dirt from the push area and rotating the master piston 180 degrees (rotation) can dramatically change the lever feel.
This can be considered a meticulous maintenance task, but I have experienced firsthand how disassembling and cleaning the master cylinder, along with performing this work, transformed the brake into something with a completely different lever operation feel and touch. Immediately after purchasing a used motorcycle, I sometimes felt a "click-click-click" or "squeak-squeak" sensation in my fingertips when operating the brake lever. After performing this work and applying grease, the lever operation feel became surprisingly different. While the effect wasn't as dramatic as with the front brake, performing the same work on the rear brake master also provided improved controllability compared to before.
If you find disassembly maintenance bothersome, or if you want to experience an immediate difference and try it out, remove the brake lever and try rotating the inner piston (180 degrees) that is incorporated into the master cylinder. When doing so, be careful not to damage the rubber boot that covers the piston. If you feel any unusualness in lever operation, try rotating the piston to change its contact position using this method, which doesn't take much time. You should notice at least some difference in lever operation and touch.
Maintenance for maintaining condition is, above all else, a vital and important routine. However, the work described here is about achieving that "slight difference" that lies one step beyond. On rainy days or during long-distance rides at night, these subtle differences can make a motorcycle feel more "manageable" and easier to brake with. Try it when you have the opportunity.
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