what would we do without u tube and google
Oil of Wintergreen is methyl salicylate, used to treat insect bites and stings. You can get it from the chemists. It's chemically related to aspirin, though don't try using aspirin instead, to soften the rubber.
Also see:
http://www.kawtriple.com/mraxl/tips/rubbersoftener.htm
https://en.wikipedia.org/wiki/Methyl_salicylate
Electrolube RRR may be worth a shot. Some rubbers it'll soften to a degree, some it won't. If yours have gone hard, I'd think owt you do is a temp fix at best.
Wipe some household mastic round the mating surface and bung them back together before it sets.