If anyone has stepped into the realm of repairing (or completely re-tubing) trombone playing slides (but has not yet encountered a Bach "Stradivarius" playing slide) there's a particular trick to installing a new upper-inside slide tube (requiring altering parts as they are installed), as well as its mouthpipe/venturi tube.
I guess (??) there could be alternate methods (though they would be difficult for me to imagine), but (in my estimation) if that particular tube (and mouthpipe) are not installed a certain way, "frowning"
may well occur. I'd be glad to talk anyone through it (telephone), but please don't just call "in case" you need to know "sometime in the future".
Call, though, ANY time, that you NEED to know how to do this (for an ACTUAL repair that's on your bench).
hey...I can also tell you how to ship a repaired trombone playing slide. (psst: "Slide boxes" are DUMB.)
no...I'm not a "know it all"...but I AM a "know it SOME", and I'm glad to share what I DO know.