honestly I think your compressor is toast and or you don't have the valves on your gauge open.
youtube a quick video on the gauges.
do you have a vacuum pump? if so just spend the 200$ or whatever they are up to these days to get a new compressor, but not a auto parts store. get a sanden, they made a drop in for these trucks, they aren't rebuild and does away with all the failure points. check their website, you can order directly or find the part number and get it someplace else but don't get a knock off sanden design. get the real thing.
Sanden, Sanden USA
www.sanden.com
it's always best to replace the accumulators/dryer that's on the fire wall and flush the system, but most times you won't be able to. the lines will be frozen up and you'll twit the tubes off the evaporator behind the dash. you'll have to remove the dash to replace that. the orifice tube is worth replacing if it will break free, they usually do but if not. to me not worth damaging the whole system. just throw the compressor on, vacuum the system down and charge with the correct amount of r134 and you'll probably be fine. that compressor is bullet proof and stuff that will destroy any normal compressor just doesn't kill these. in my experience it out lasts the truck.
unless you got time and money to replace everything, if so definitely do it.
this looks like a leaking body seal to me.