i posted a similar question as i did here - but since i have no memory left, i can't remember where i posted it.
one of the responses was:
"The reason for this being that the older drums by virtue of the rubber seal in the piston becoming less "supple" needs a thin film of "Vaseline" to form a water tight seal to prevent leaks out the back end of the drum.
This works for the most part, but care must be taken NOT to use more than the required amount of solution for the reel size and the number of reels used, and to store the piston in the drum in the relaxed position so as not to compress the seal while in storage."
i hope this helps!