A/C operation problem: any ideas or?

[Coley]

...I'm not exactly sure where this thread is best served but regardless here is my issue.
The a/c in my truck used to work and I believe it may have been converted at some point to a newer type refrigerant
Regardless, right now when I turn it on, it doesn't blow particularly cold so I checked a few things under the hood.
Everything appears to be operating properly (mechanically, etc) including the compressor, clutch, vacuum control points etc.

However....the piping near the firewall and the main a/c firewall box has an interesting condition.
the lower, small 'u-pipe' is very frosty and quite cold to the touch. However once it gets up to the 'expansion valve'?? it goes into a larger, upper U-pipe that then curls into the main firewall unit. This larger, upper U-pipe isn't cold at all.
A lot of the crew here are really up on the a/c systems so I thought I would ask if there are any ideas as to what might be going on with this system?
...here is a quick pic of a system typical of what I have under my hood. I've tried to mark/identify these 2 different pipes in the drawing so hopefully they show up ok.
...any ideas, etc from the a/c experts are more than welcome.
All good
Coley