The hissing is kind of normal when vacuum is released. Air hissed into the can, but air also escapes around the rod too. I could see a stiff/hard wiper or diaphragm causing drag.
Old rubber-band-powered airplane guys used to soak their rubberbands in simple green for a few minutes. Apparently it worked well to 'revitalize' the rubber to some extent. Haven't done a bunch of it myself, but it's worked when I've tried it. Following up with o-ring grease should help too.
Another suggestion I've run across is to use Honda silicone grease (Honda Genuine 08798-9013 Silicone Grease) for hardened rubber. I bought a tube last year, but haven't gotten around to using it - but lots of detailers swear by it.