So your cassettes don't see any light between them being filled and being inserted in the camera? Does that mean you're attaching the leader to the takeup spool in the dark? If you're doing it all-dark, maybe you could ignore the cassettes entirely and just use the spools as long as they don't unwind themselves.

They do say the cassettes get leaky over time. Have you tried a different/new cassette? Which part of yours is leaking?

I think the 2405 is without antihalation backing (certainly doesn't look like it has one!) and probably intended for direct loading of 150' and 600' rolls into surveillance cameras.