Probably the heat is gradually breaking down the rubber of the o ring, and it maybe starts to get harder. It then is not effective at sealing as its not as flexible as it was when new.
For what they cost, change them every fortnight or even more.
I used to use Extracta hand tools, and eventually as the bore where the plunger fitted into, wore down, regular as clockwork, a new o ring, and a new spring occasionally after rusting and snapping.
Dave