…just been through this whole exercise with ‘750 intake rubbers, one of which had become distorted in situ. Hot air, long soak in Wintergreen did soften them…. temporarily.
Given I had the bank of carbs out and the poor seal the distorted one was making, I just bit the bullet & bought a new set.
If you’re talking CB400F, good luck with getting the airbox back with any reluctant rubbery bits!