On the city water inlet if you remove the strainer you can see the end of the check valve (and its spring). I'll usually just gently push on it with my finger until water flows out. Its hard to do with the water system fully pressurized so I'll run the pump until it stops then crack a faucet for a little bit to relieve some of the pressure.
I couldn't find a picture online of it, but this website does describe the feature (and show someone pressing on it, step 6):
http://www.recworldrv.com/index.php?pid=320
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