Tape all over a broken solder joint, where the bell connects to the valve casing. Was it air-tight? Haha no, not at all.
How it looked underneath all the tape:
Insert youtried.jpg here.
Someone didn't properly clean and flux the area, so instead of flowing into the joint the solder just lumped up around it.
I removed the bell completely, removed the excess solder, rolled out the dents in the bell flare, straightened the bent braces, cleaned out the joint surfaces, realigned everything and soldered the bell back on.
And how the soldered area looked after a quick polish:
Not amazing but a lot better than it was. Ancient, beat-up school instrument, and they just needed it functional, not pretty.