SCORE!
Tom,
Some data:
1. Stork Custom 5 Mouthpiece - Never heard of it. The YEP-642 is a large shank horn, so you can use any Bass-Trombone shank mouthpiece you'd like. What were you using on your Besson?
2. 2nd Valve slide falling out - This is a common problem. I used to use a rubber band to hold the 2nd valve slide on my old Besson 967 because I was using Vaseline instead of a good slide grease. Use Hetman slide grease. This stuff is VERY sticky! Also, consider having a technician ever so slightly wedge the slide out a bit to give it just a tad more friction.
3. Micro-abrasion on the silver. I had this happen on my Besson 967. The problem was that I used Hagerty's Silversmith's Polish (liquid) instead of the Hagerty's Silversmith's Spray Polish. The thicker liquid polish is much more abrasive than the spray on stuff. Also, Brasso is a big time "no-no" on these horns. To get my horn back in shape, I used Wright's Silver Cream highly diluted and polished very carefully, several times. As long as you don't rub too hard and long in any one place, you should be able to get the horn to look quite a bit nicer. On an on-going basis, I highly recommend Hagerty's SPRAY (not the liquid!)
Doug Ruby
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band