Delete Write Cache on Shutdown

Hello WSM users,
on a WSM 4 Headquarter I checked the option "Delete Write Cache on Shutdown" on System Settings, wich says "If this check box is checked, write cache files for volatile partitions on devices are deleted when devices are shutdown". This setting is correctly propagated to all Linked Sites, but it doesn't work. When I turn off devices, the write cache stays on the streaming directory, and it is overwritten when device boots up.
So, when I deploy a new patch for an image, old write cache files stays on the directory and new files are created: this is causing problems because I have to manually delete write cache files every now and then before the disk-space utilization becomes critical.
Can anyone help?

Thanks in advance,
SS