There is a check valve in the inlet that is supposed to prevent water from flowing out when using the pump. Sometimes, this check valve will stick open or sometimes a rapid change in water pressure like you may get from a quick-turn ball valve (as opposed to a spigot where you turn the handle several times to get the water flowing) will blow the O-ring off the stem inside the check valve.
There is probably a little conical screen inside where the hose connects. The first thing I would do is pry that our and see if I could reach the stem and re-seat the valve. If that doesn't work, some check valves can be disassembled for repair, some can't. If you can't repair it, you may have to replace it.
2015 DRV Tradition 375KPS
2015 Chevy Duramax 3500HD LTZ Dually
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