If you have the valve in the proper position, and the jug is lower than the pump, it may not prime.
While it 'should', sometimes it doesn't.
Either hold the jug higher or pour some antifreeze in the hose you connected.
One other thing... you mentioned the valve was made to attach a piece of PEX on it. It could be... but most folks use a piece of braided or nylon hose and a hose clamp. If you are using PEX without a proper PEX clamp, you are almost certainly sucking air at the valve.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel-Goodyear Endurance Tires
2017 Ford F-Series SCREW 4x4 - Factory Puck B&W Companion
TST Tire Monitor w/Repeater - Sinemate 3500w Gen.
Days Camped: '18=51 ☼ '19=58 ☼ '20=110 ☼ '21=21