I had previously said that my technique for clearing the city water check valve of fresh water when winterizing was to "burp" the check valve with you finger.
It has worked for me for several years.
Don't do it.
Monday I broke the check valve by pressing too hard. The rubber rolled off the plastic plunger.
I put a new one on (Amazon Prime rocks) today and we are good to go.
Took about 10 minutes to swap it out; grrrrr.
Also ordered a hand pump (10 bucks) to pressurize it properly in the future.
Amazon.com: Camco 36003 Hand Pump Kit: Automotive