1967 K20 Transmission vacuum/breather line question?

[nchandler]

Hello all,

I recently picked up 2 – 1967 chevy trucks which have been pretty bastardized and mistreated.
Currently I am working on my K20, tightening up bolts, cleaning and just trying to make sure it is in decent health before I start driving it around.
It was shifting hard so I changed the modulator valve on the transmission and I noticed a vacuum line or breather hose on the top towards the back (see pics below).
Anyone know what this is for and where it should go?
Thanks in advance.