A/C Compressor - Please help diagnose (lots of details for experts)

[BruthaMan]

Guys, I need some help diagnosing an ac compressor issue. Please bear with the information below. Trying to make sure I document what I've done, so those with experience on this have as much information to determine a cause and possible solution.

Some background:

I have read, read and read for weeks (in spare time), posts from this forum and other auto-ac sites. I have absorbed information and learned (which was my goal...I make good money, the decision to learn this was not one from saving money, but rather a desire to learn) a great deal about AC systems that were for years, a big mystery to me.

When I bought my 72 a few months ago, the previous owner mentioned that the compressor on the truck was new. It has the black metal seal over the ports in the back. I'm sure that it was a new compressor.

I ordered a new hose kit, and all new hoses, including the aluminum hoses from the condensor and dryer. I flushed the entire system, thoroughly, multiple times as it had been open for an untold amount of time when I bought the truck. I have installed a new condensor, new dryer, new expansion valve, new AC Switch, relay and high speed resistor (since we gutted the entire truck AC system to clean out 40 years of dirt, leaves and debris). Pretty much everything is new except for the evaporator core (which was fine, though we cleaned it good) and the POA valve.

I removed the POA valve, cleaned it good (flushed it) and built a POA valve tester, hooked it up to the tester with 60 lbs from the compressor in my garage, and adjusted the POA valve to 29.5 PSI (per instructions on the autoacforum.com).

In short, I have meticulously prepared for this. I have all the tools and I've probably taken more time than most would because I really want to learn this and make sure I do everything correctly.

First Attempt:

With the first attempt, I tried out Auto Refrigerants ES-12a. I used Ester oil on this attempt. We got it to cool some, down to about 58 degrees at the evaporator core (using a digital temp gun). I was concerned about pressures at that point (using a gauge set on high/low side to monitor pressure).

Up to a point, the compressor was fine. I took it one can more and suddenly the compressor started squealing. Not the belt, the compressor. And it would start "locking up" some, where it was struggling to turn.

At this point, after doing some reading, I thought maybe the clutch had given out. So I replaced the clutch.

Second Attempt:

Next, I flushed the system entirely again, thoroughly. I flushed the compressor, thoroughly, in preparation to use mineral oil (used 11oz of mineral oil). I came across a large stash of r12 cans locally and figured why not use what the system was meant for. I bought another new drier since I previously used ES-12a. Figured it was a safe bet for going to R12.

Pulled vacuum for 45 minutes, let site for about four hours, no leaks. Started adding r12. Engaged system, started seeing the compressor squeal again when A/C was engaged. At this point I figured it had to be the compressor that was bad.

So I found an NOS AC/Delco compressor for our trucks on Ebay. Great score. It was minus the clutch (original box and instructions) and I just happened to have a new clutch that was put on the previous compressor and used for maybe five minutes.

Third Attempt:

Uninstalled the new clutch from the old compressor and installed it on the new AC/Delco compressor. I drained the oil from the compressor. I wanted to be sure it had the correct amount of oil in it. Added 8oz of mineral oil to the compressor and 3 oz of oil to the drier.

Pulled vacuum for 1.5 hours. My home AC guy was at the house today and said it wouldn't hurt to pull vacuum for longer. Also recommended to pierce the cans first, let a little start coming through, THEN hook up to low side on POA valve so no air would get into the system.

I picked up all new belts from Napa. Figured we were here, might as well replace all the belts. We replaced them all. Snug, not too tight. These are the good belts.

So, after four hours to make sure there were no leaks, we started off by adding one can of r12 to the system. Next, we started up truck, put fan on high, engaged AC.

The heart of the issue:

SQUEAL!!! It's now doing the exact same thing as before. The compressor is squealing the instant the AC is switched on. It acts as if the compressor is just not able to turn. The belt is tight enough, but not "too" tight.

I'm at a loss. How does the same symptom occur with two different compressors and two different clutches (I don't believe the old clutch was bad)?

The belt is on the right pulley, it looks straight when looking at it from the side.

We hooked up a voltmeter to the A/C power pigtail. Was getting 11.5 to 11.7 volts when it engaged.

Is there anything else I could be missing? I've spent a good month on this, between waiting for parts to get here, trial and error, etc. It's like everything is perfect, except that the compressor is squealing and not wanting to rotate.

I should mention also, there is no problem when the A/C is NOT engaged. Belt/pulley turn fine, not issue. Only when the A/C is engaged.

A brutha's gotta have his A/C. I don't do well in the heat and I live 10 miles from hell.

Recommendations?