I think either your check valve isn't seating properly or possibly got damaged over the winter. Try releasing the pressure on the water line. Then press in on the check valve release (it's inside the the water filler inlet. You might have to remove a small screen before you can get at it) and have someone inside turn the pump back on. Release the check valve when the water squirts out.
The last step of the winterization for the fresh water line is to turn off the water pump and release the pressure on the water line. Then (this is a two person operation) the person outside pushes in on the check valve and has someone inside turn on the water pump. When the water squirting out turns pink, release the check valve and allow the water line to pressurize again.
If you didn't do this step, there could be water at the check valve instead of antifreeze, and if it froze, the results are unpredictable (as us old-time computer professionals used to say).
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TV: 2009 GMC Sierra 2500HD | Crew Cab | Std. Box | 4WD | Duramax/Allison
RV: 2000 Jayco Eagle 266 | FBS | TT -- 1986 Coleman Laramie pop-up -- Still in the family!!!