Thought I'd give it a try here.  The dedicated ranger forums I've found aren't very helpful.  

I had an overheat a few weeks ago and this valve on the low side of the compressor started leaking.  From what I've found online the second one in that you can't really see is the pressure relief valve, so I have no idea what this is.  

It has some kind of threaded center that I could turn back and forth a little bit but I couldn't really get anything on it without taking other things apart.  I was going to put a new charge in it but there is no point if it's going to start leaking again.

Looks to be another relief valve you can change them without changing that whole damn line. But you will need to find a good parts guy at ford to get them. Some places who deal with aftermarket A/C systems will stock those for buses. Check them out

It seems logical considering it was leaking after the overheat I just didn't figure there would be two separate valves to serve the same purpose.  Maybe I'll take some things off next week and see if I can get the center part out and see if there is some kind of o-ring that seals it that can be replaced.  

I might as well hit up the salvage yard and see if anything is there also.

ETA:  Took a quick look because I couldn't find a wiring diagram online but there is both a high and low pressure switch in the system.  I thought maybe the PR valves were a fail safe instead of a HP switch but I don't see why they are really necessary with a HP switch.    

Also both lines runs to that block on the back of the compressor which I didn't notice before.  So there is a pressure relief for both the high and low side of the system.  That doesn't seem necessary either.  Worst case I don't see why I couldn't just remove it and plug it with something else, anybody see any issue with that?  

I'll assume they don't even put PR valves on cars anymore.  This is the first time I'd had to deal with one and I've done a decent amount of A/C work on vehicles.

The "block" is a manifold.  Take that off and there should be two green o-rings sealing it to the compressor.  The two bosses on the manifold are for the high pressure switch mount.  I would guess the "valve" is just a cap to seal off that boss.  The manifold can probably be used in multiple vehicles and the pressure relief can be mounted on either boss depending on the configuration.  If the sealing cap is now leaking you may need an entire new hose/manifold assembly.  

The low pressure switch or cycling switch energizes the clutch and couples the compressor pulley to the compressor shaft.  The high limit switch should de-energize the clutch.  The pressure relief is a fail-safe and will open when over pressure conditions occur.  Given its age it may not reset.  I believe the cycling switch is on top of the filter/drier low-side line.  It looks like you are missing the top right mounting bolt to the compressor.  If so it may explain why you now have a leak at the manifold connection.

I'm not leaking from where the manifold seals to the compressor.  It was venting refrigerant out of the top of the pressure relief valve (or what I assume it is) that's sitting on that manifold.  

I did notice a bolt was missing.  I'll have to track one of those down at the junkyard hopefully.  I've had to fix a lot of half-assed repairs from the previous owner.  

Just looked it up and an aftermarket assembly is only like $60.  Maybe I'll just go that route instead of screwing with it.  Probably just weigh in a new charge for the time being and order the new part.  

Thanks for the help.  

I understand where you stated the leak is.  Look at my last sentence of my first paragraph.  That sealing cap or plug is loose if you can turn it back and forth as you describe, it is no longer a sealed plug.  You might be able to remove it and thread in another plug.  Or replace the hose/manifold assembly for the $60.00 price you've found.

Do this.  I've been in the same boat, and it's easier to change out the hoses than to play around trying to fix something like that.
Change all of the o-rings while you're at it, and maybe even the receiver/dryer.
When you're done, all seals will be new and you won't have to worry about leaks for a few years.