That's pretty much what I expected. In that case, I'd be inclined to use some sort of strong bonding agent (glue -- probably an epoxy). Or better, if it's possible, actually work the joint free and glue it solidly. That would restore structural stability in addition to eliminating the leak whereas sealant applied to the leak is just a (temporary) surface fix.
Of course, I don't know if the instrument permits of that approach to the joints. If they're really junky to begin with, it may not be possible to re-bond them. But then (largely as an act of desperation) I'd resort to gluing over the leak and entire joint with epoxy -- assuming that epoxy would bond to that plastic (for some plastics it won't).
Or hot glue. Or (if I had the tool or wanted to invest in it), plastic welding.
Just some thoughts on trying to fix something that may not be fixable.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)