Fuel Tank Vapour Pipe Remove Or Block Off?

I read this post, because I'm also interested in putting in a new canister from another car that's smaller than the original. I had a close look at the existing one that's mounted up just behind the battery on the battery hold down bracket on my KE55. It has the two (2) hoses on the top clearly maked "to tank" & "purge" which connects to the outside base of the carby. There is also a hose directly out the middle underside of the cannister that goes through a grommet into the chassis rail directly below the cannister. I presume this is some sort of final overpressure bypass if the cannister becomes blocked & doesn't release the fuel tank pressure. Can anyone confirm what this third line does ?

Cheers Banjo

Edited April 14, 201412 yr by Banjo