Yep, I was thinking that careful use of a heat gun might not damage the rubber ... probably depends on the heat shrink ... I've seen two types .. the shiny stuff and the dull stuff ... forget which shrinks easier ... I think the dull stuff.
I just bought three cans off ebay ... one is probably a re-pop, the other two look like pretty nice originals ... says none of them hold vacuum ... I bet the problem with all of them is that seal. The OEM units probably have rust somewhere under them.
https://www.ebay.com/itm/183244499481