The RCB S1 Piston 14mm is the official internal repair and maintenance component for the RCB S1 Forged Master Brake Pump. It is a precision-engineered kit designed to restore the hydraulic pressure and “snap” of your master cylinder if it begins to leak or feel “spongy.”
Unlike universal kits, this is a dedicated Forged Series component, ensuring it meets the high tolerances required for radial-pump performance.
Key Features
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Forged Compatibility: Specifically designed for RCB S1, S2, RS-1 & S1-FL Master Brake Pump series.
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High-Grade Material: The piston is typically crafted from lightweight aluminum with a specialized coating to reduce friction and wear against the master cylinder wall.
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Complete Seal Set: Most kits include the main 14mm piston along with high-pressure rubber seals (O-rings/cups) and a return spring to ensure a complete hydraulic seal.
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Restores Performance: Over time, brake fluid can degrade rubber seals. This kit is the standard solution for fixing a “leaky” master pump or one that is losing pressure under hard braking.
When do you need a Piston Replacement?
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Fluid Leaks: If you see brake fluid seeping from the area where the lever pushes into the pump body, your piston seals are likely worn.
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Spongy Lever: If you have bled your brakes but the lever still feels soft or moves to the handlebar with little resistance, the piston may be allowing fluid to “bypass” the seals.
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No Return: If the lever is slow to return to its original position after you let go, the internal return spring or piston surface may be compromised.
Comparison: 14mm vs. 17mm Piston Kits
It is vital to use the correct size.
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14mm Kit: Used for the standard S1 pumps designed for 1-piston or 2-piston calipers (common on underbones and 150cc–300cc bikes).
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17mm Kit: Reserved for the high-volume S1 pumps used with 4-piston calipers or dual-disc setups.
Maintenance Tip
When installing the new 14mm piston, always lubricate the new seals with fresh DOT 4 brake fluid before sliding them into the bore. Inserting them “dry” can cause the rubber to tear, leading to immediate leaks.
Safety Warning: Rebuilding a master cylinder is a critical safety task. If you are not comfortable bleeding brakes or working with internal snap-rings, it is highly recommended to have a professional mechanic install the kit for you.
