little trick if your too low on coolant and you don't want to blow hot steam in your face. Take your reservoir take and fill it with cold water. Flip it upside down and squeeze the water into the radiator. will cool of faster so you can pop the cap sooner than later. That it if your reservoir is removable. Mine in on my honda is..It just clips into place.
I would think, if it's out of water, put water in the reservoir like you said, and it should suck it up by itself.
Also, if you are near a hose when you're overheated, spray water on the radiator to cool things down so the cap can be removed.
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