Len you don't want to put anything with wax, oil grease, certain types of soap into a part that will contact the water the gold is in. These things will cover the gold and make it float out of your sluice or pan.

So ...... Let the cups dry out in place? And use nothing?

Yep. When you use the next time soak it for a few minutes and it will soften back up.  Alternative is a rubber ball, with a washer on each side and a wing nut to adjust the fit in the tube.

 

Some are also made with a semi-slip fit pair of tubes with a rubber "o" ring in a groove on the inner tube.

To prevent cracking and to hold shape, I use vegetable oil on leather cups for long term storage but do NOT dry out cups for extended periods of time like months. Keep leather cups slightly damp for short term storage like days and weeks but do NOT soak in water. When leather cups are removed from long term storage you can remove the vegetable oil from leather cups with mild detergent and rinse completely. This should be done a few days before use to allow the cups to swell slightly and become limber. Make sure ALL vegetable oil is removed on your leather cups or you will have disaster like Tim Leibel described and your gold will travel downstream. I do not recommend using rubber as leather is a natural lubricant with plastic. Check out Lehman's in Ohio. I just ordered my second set of preformed leather cups from them this week. My old ones are about four years old and not in bad shape, I just wanted new ones for my second hand dredge. Lehman's also has instructions on how to care for leather pump seals so you can get it straight from the horses mouth. Good Luck and please keep us updated.

Here's a big THANK YOU! to TIM LEIBEL, ARTHUR WAUGH & GARY WHITED for their thoughtful and insightful help.Been up to my "Yarbels" twice in freezing, frozen creeks in wisconsin this year, seeking glacial dust with a scoop in cobbled creek bottoms. It'll be nice to know my Gold wount be heading downstream when I try my new slurp gun after prying those stones out of where they've been resting for who knows how long. Best regards, Len Solarz