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Trueweb,
From your description, I think you may have one of the combo sweep/bulb type systems. Mine has a separate rubber sweep near the outer part of the TT wall and a separate inner bulb seal attached to the inside surface perimeter of my inside flange. That bulb in turn, seals against a large inner angle-metal edge that serves as a back-up support to the wood trim moulding/flange on the inside.
Your theory on water following a bulb-seal makes sense. I kind of wonder if water doesn't bead up, and roll as droplets straight down, on wax-like siding, but stick to and run downhill as sheets on rubber, even sideways. An outa the box solution, but maybe waxing the rubber to repel water would help.
I think we had a contrived small "bulb" leak when I recently partially retracted my slide trying to avoid wind-blown rain. Partially retracting it tilted the entire slide as the inside edge of the slide floor rose to clear the static TT floor. My plan was that copious rain, apparently inadvertently following the folded u-channel in my failed and wandering sweep, would then run straight down the slide sidewall before getting in. But it seemed there was still a small dribble coming off the end of the now tilted, gapped and uncompressed vertical bulb perhaps because some rain was still blowing against it near the top.
Sometimes I think the only really good seal would be a continuous inflatable ring(s), after the slide was parked in place, either in or out.
I looked under my older 2004 TT slide and I don't see the plastic cups of which you speak. However there are some sheet metal formed cup-like "covers" at the bottom. They seem to extend under the area that should have the end of my inner vertical bulb seals located above them. Are we talking of a similar purpose? What purpose?
I think my "slide cups" might be purposed to close a gap to easy insect intrusion rather than catch water for instance. I do get a little lost as to what exact exterior location corresponds to an ajacent interior part. I would love to see an entire slide removal to understand this precise relationship.
On another note, I completed my top sweep (wiper) replacement
that I mentioned earlier. Instead of ferreting out an exact rare rubber wiper with a spring-loaded "U" shaped edge to attach over a flange, I bought the ordinary flat rubber wiper with pre-mounted adhesive strip and adhered/screwed it to the outer face of the top flange using two 6' x 1/2" x 1/16" aluminum strips that I picked up at Menards. The metal strip serves as a continuous "washer" to keep the bolt heads from pulling through and I screwed it every 6" with stainless #8 x 3/4 panhead sheet metal screws. It looks good (in dry weather).
Wes
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