If you crawl under and look at the valve stem on the end of the black tank dump valve, and that valve stem moves 2" to 3" or some part of that, then the slider valve should have opened.
If that is the case and no water is coming out, this sounds like a clogged tank discharge port.
Is your dump valve straight up from the end of the 3" dump connection or is there a 90 degree elbow there? Straight up makes this easier to unclog.
See this post it may help http://www.sunlineclub.com/forums/f7...tml#post135863
They show how to make a blaster tube you go in with to break up the clog. You may need to go up the dump pipe with the blaster tube. Since the tube is like 1/2" OD or so, hopefully it will not all break loose at once to allow you to get out of the way when it does break loose.
Good luck and let us know how this goes.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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