1991 F150 XLT Lariat 4.9L with factory AC. Somewhere, I assume, the A/C system was converted to r134.

Regardless, I had working A/C for the last year then, suddenly, the coldness ran out. Aside from the basic, that's the compressor, that's the condenser, it needs freon, I know little about automotive A/C, but I do enjoy messing with stuff, hence I own a 91 Ford. Rather than just trying to push some r143 into the system, I got a set of A/C manifold gauges and hooked them up. Turns out I have zero pressure at all in the system, thus zero refrigerant. Got my hands on a vacuum pump and tried to pull a vacuum. The system wouldn't pump down past 5 in Hg. I must have a leak. Got a UV lamp to see if by chance the oil/fluid had UV additive and found a leak around the high side schrader valve. Replaced it, still won't pump down past 5 in Hg.

With nothing to loose at this point, I disconnected the condenser and plugged up both hoses leading to it and tried to pull a vacuum. Nothing. Hooked the condenser back up and removed the block that has both hoses going into the compressor (image included). I can now pull a vacuum down to just under 30 in Hg. This means to me that the compressor is the culprit and is leaking.

So, here's what I'm wondering...

How can a compressor leak? is there a valve that I'm missing? Are there seals inside? If the coompressor is bad, does it no longer hold vacuum? Any insight here would be appreciated.

Aside from my issues above, how should I proceed?

 

I think there’s seals on the front of the compressor since there’s a shaft or something going out through it to the clutch. There’s the seals where the hoses go in. I’m not an expert that’s the ones I know of though. Try using the UV light by the compressors connections and the front behind the pulley.
I do know if you take the compressor off you need to pour the oil into a measuring cup and put that same amount of new oil in the new one. Assuming you need to replace it

 

It's a ceramic seal and, if it breaks, it will leak big time.

 

You could start with all replaced orings and quality 134A conversion fittings, if you go with 134A. The cheapo ones are crap.

Hoses can leak at the crimps. The compressor does have a shaft seal and can leak there. Yes, it can leak at the manifold with the suction and discharge orings. Your condenser could have a hole from a rock. Your manifold gauges could be bad also, unless good brand like Mastercool or Yellow Jacket.

Being that these components are all 30 years old, you really should just start fresh including a small tube condenser or do a parallel flow swap from a '94+ truck. You just need to change the liquid line fitting from a No 8 to a No 6. Super easy.