Emptying Liquid Reservoir

[BluerGoop]

My brain doesn't really work as well as it use to, so I'm having a hard time wrapping it around emptying a liquid reservoir when it's full.  I tried a not gate to turn the signal green at 100% instead of red, but the low threshold really seems to be messing up this application of automation.  The reservoir automation seems more concerned with the incoming stream.  I'd like to make it worry about when to turn on the outgoing though.  Is there a way to do this?

[Yunru]

Put a mechanical airlock under it, such that when it's open the reservoir is disabled.
(A disabled reservoir still accepts fluid, but doesn't distribute it.)

[Soulwind]

The automation signal from a res isn't used to start or stop flow.  It can only be used to signal another item like a mech door under the res or a pump feed into the res

[BluerGoop]

I figured it out but thanks for the replies.  I hooked a liquid shutoff to the outgoing pipe and a secondary route.  I then used the not gate to turn the red signal when full to green.  That only lasts a moment, so I used a buffer gate after the not to keep the shutoff on for a good amount of time.  Seems to work!