I suggest that you watch the video on the website of HIS GLASS WORKS in Asheville, NC. They show how do the process with HXTAL which is the epoxy that museums around the world use as it is the most archival and never yellows. I went there and watched the process and came home and made my own vacuum pump using the Harbor Freight hand brake pump and a mason jar which with the hose and connector which is about a $30.00 solution. When I pump it up and start seeing the bubbles coming from the rock, I cannot say that I loose vacuum for quite a while. If you are loosing vacuum, I would check to make sure that the connector in the lid of the mason jar is sealed totally.