The water gets there because brake/ clutch fluids is hygroscopic.
Hygroscopic means it absorbs moisture from the air.
Initially it mixes with the fluid, but as it gets more concentrated, the water eventually pools.
If there are leaks in the seal on the master cylinder cover, the moisture is sucked in. Any air above the fluid dries out, which shrinks its volume, so more air is sucked in.
That's why it's important to change brake and clutch fluid at regular intervals.
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