Anyone know what the black line running under the brake vacuum booster house is? I noticed it wasn't connected to anything! Looks like a vacuum line. This is just to the left of the booster.

 

Looks like a breather

 

Yes, breather line. There are several of them .

 

It's a breather tube for the front axle housing, the transmission or the transfer case.

All three have nylon breather lines that terminate open ended in the engine compartment near the firewall. The RAVE shows the ends of the lines for the axle housing and trans running vertically with a 180° bend at the top, so this one may be for the t-case.