What is this thing?

Bleed port for antifreeze coolant.

 

Where the repair manual explains refilling the inverter and transaxle cooling system, it will show you how to use that.

Toyota Service Information and Where To Find It | PriusChat

 

Specifically it's a fitting to bleed air out of the inverter cooling system. You would take the cap off the inverter coolant tank and fill it. Then attach a small hose to the bleed screw and secure the other end in the opening of the tank (the hose will try & jump out). Turn the car ON (the inverter pump will run) and open the bleed screw.

The pump pushes coolant and any air into the tank (keep it full). Eventually all the air is purged -you can hear the pump get much quieter- then close the screw.

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Good idea to hold the base of it with a vice grip when you try to loosen up the bleed valve as its really really tight from the factory. You will notice as you try to loosen that valve the whole assy is bending.

You will know the system is properly bled & bubble free and working well when you see good circulation in the inverter coolant reservoir
and the pump is quiet.

My coolant tank looks like its boiling it has so much movement. I checked the inverter coolant temp with an IR and its running about 100 degrees cooler than the ice. And an increase of 10 degrees cooler than stock using Redline Water Wetter.

When I changed my Inverter coolant I also add a bottle of Redline Water Wetter. I also use it in the ice rad too. Really good anti corrosion product and eliminates surface tension which means no bubbles. And its the same red color as LLC.

Anti corrosion improvement is a good idea as we have seen many clogged inverter coolant channels from lack of maintenance.

Used Water Wetter in both coolant systems for 12 years. Rad looked brand new inside 12 years later.