A few years ago most parts stores had one on the shelf and I was scraping cores by the truck load. Then all of a sudden they were out and need your's to rebuild. Now I save the cores. I've been wanting to learn how to rebuild them and maybe keep some on my shelf. I've used Firm Feel with good happiness. Last year I sent one to a shop over in Denver for rebuild. Was to be a one week turn around. Well took over 2 weeks and then blew a seal in about an hour after my customer got the car home.