The cup (or lip seal) is a wear part. Its role is to transform the pressure exerted by your foot into hydraulic pressure into the pipes.
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Symptoms of fatigue: "Spongy" pedal (which goes down to the floor), liquid leakage at the bellows (cap), or braking loss during high heat phases.
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The material: These joints are EPDM (ethylene-propylene-diene monomer) to withstand the brake fluid. NEVER USE STANDARD JOINTS Hardware stores, they will disguise in a few minutes in contact with the liquid.