The OS X Yosemite installer is Mac App Store content, which is cached by the Caching service in OS X Server (Mountain Lion) and later. You need to stop and restart the Caching service in order for local clients to use your cached copy of this installer.
OS X Server: Caching the OS X Yosemite installer
To cache the OS X Yosemite installer, stop and restart the Caching service in OS X Server.
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