The check valve is a common problem. Many posts on this forum with solutions. FYI. on most the check valve is built into the pump. Common solutions are to add an external check valve or replace the pump.
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DrLewie
Flemington, NJ
Former: 2007 Cardinal 30W LE
Now:2018 Arctic Fox 27-5L
2008 Ford F-250 Long Bed Diesel
Total of 30 Months on the Road Since 2008