How can the fumes from perished breathers be so stronG?

[futurecut]

It's been absolutely honking! I thought they were ok but i stopped and waggled them a bit and they looked like if i buggered about with them they'd fall apart. Didn't see any holes or unsealed bits though. How could the smell get so bad? Can a gap develop at the top of the filler tube at the grommet? Think i'l look there tomorrow.

[Mocki]

also check the overflow for the filler, the pipe that runs alongside the main filler pipe, mine fell off !