Run systemd service before unmount

I'm trying to run a systemd service at shutdown: it kinda works (it gets called) but it runs after filesystems are unmounted, which is not what I need. So I tried with Before=umount.target and WantedBy=umount.target without any luck (filesystems are unmounted before running the service). When I say "filesystems" I mean all but the root. Is there any other target to pay attention to?