unmount mount point when exiting gnome3 desktop

[John Naggets <hostingnuggets@gmail.com>]

Hello,

I would like to automatically unmount a DavFS mount point
/media/webdav) when a users logs off the gnome3 desktop on jessie.

For that purpose I have added the following command to
/etc/gdm3/PostSession/Default:

/bin/umount /media/webdav

But it looks like this script never gets executed when a user logs off
as the partition is always mounted. Do I need to do anything else?

Cheers
John