Siphon / Anti-siphon
No, all shunts do not have anti-siphon valves and there still remains some controversy as to whether they make a difference.
In theory as you rise from recumbent or seated positions the shunt will "siphon" because of the sudden pressure change with elevation
. These devices are supposed to prevent that and thus avoid over shunting, ie, draining too much fluid with resulting disturbances.