I have had similar issue but when it rains steady. I have checked every place possible on the cap. My last attempt was caulking all visible areas around the gutter and channel. It hasn't rained since I caulked. Just waiting...
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2018 Eagle 30.5 MBOK, L-dinnete, orbital pinbox, Husky 16k W, 2016 Ford F-250 Lariat w/tow pacakge, Crew, 6.7
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