A simple radiator bleed key.

Works well with PETG, if you only have PLA then make sure to use 100% infil. 

I added the key ring hole in case people may want it on a large key ring, but it may be difficult to fit onto the key ring. 

The key its self may wear out after a few uses, so you may have to make multiple or use a more durable filament like PETG (If you are using PLA.)