What a brilliant question...... I'm sure Kev will be along (or one of the other mechanical geeks) to give you the full breakdown, but in simple terms;

The reservoir feeds into a valve(piston) that opens, then closes just before the pressure builds, so yes you have your closed system, but the reservoir and feeder pipe are not part of it at the time the piston starts to build pressure.

Like I say, you'll get a full and propper explanation soon....

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