07 columbia ac leak

inside the firewall. it's called evaporator. sounds like your simply talking about a o-ring seal though. to which you take the connection apart. and simply replace the seal. they should be green. and can be bought at any auto parts store. usually as a kit though. i've never been able to buy just one.

make sure to soak the seal in oil designed for 134 systems before installing or you may end up tearing it when putting back together. and don't forget to EVACUATE the system before recharging. sucking all the air and moisture out will make your A/C work much colder. they don't work so hot when there's atmospheric pressure in the system compressed with freon.

 

Also, your truck will have a receiver-drier (IIRC, all of these Class 8 truck AC systems use a TXV)... you'll have to replace that after you open up the system. It contains a desiccant which prevents refrigerant that is still in liquid form from traveling to the compressor, and it will become fully saturated within minutes of exposure to atmospheric air.