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If the top flange on the slide isn't going flush to the wall , then the first thing to do ; is check that something isn't pinched between the flange and the wall .
Run the slide 1/2 way in and look for any debris that could be stopping the slide .
A friend had a pin cone stuck in the slide seal that caused his similar issue .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.