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Is this a slide valve located upstream of the sewer hose connection or a cable-actuated slide valve located at the gray tank outlet under the bottom cover? If the former, remove it and service it. If the latter might grab the handle with a large plyiers and with 2 people give it a good pull. That worked to open a stuck black tank dump valve. Eventually replaced the cable-actuated valve.
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2022 Jayco Pinnacle 36SSWS / 2016 Ford F-350 6.7L diesel crew cab long bed 4x2 DRW
2022 Thor Palazzo 33.6 diesel pusher / 2021 Chevy Equinox LT AWD toad
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