Any years A/C question

[Cadman-iac]

Quick question here about evacuating an A/C system.
A little background, system was working just fine, R4 compressor began to seep oil out around the front hub where the clutch attaches. System has R134.
Drained the system, re-sealed the compressor and reinstalled it. Did a quick evacuation on it last night to check for leaks, and it was sitting at 28-29 inches of vacuum, the temperature was about 75. This morning it was down to about 20 inches at about 10 am. Weather is hot here but it cools off decently at night, 90+ daytime, 65-69 overnight.
So this morning I checked all my connections and evacuated again,it went right back to 28-29 inches. Stayed there for an hour and 45 minutes.  Now it's noon-thirty and hot and the vacuum is just beginning to drop, it's at 26 inches.
Here's the question. Can residual refrigerant cause a drop in vacuum as the temperature increases? I capped all the lines I opened when I removed the compressor and orifice tube.
The system was only down for a few hours while I worked on the compressor. While reinstalling the compressor I knocked off one of the caps I used to plug the lines with and I could hear more pressure from the line/system. I just figured it was the residual refrigerant still trapped in the oil that was slowly escaping and building pressure in the system.
Or am I way off base here and I need to locate a leak?
Thanks for your help.
  Rick

Edit:
 Evacuating it now for a couple, maybe 3 hours to hopefully get any residual refrigerant out of the system, and I'll see if it'll hold the vacuum overnight.
 I'd still like to know if it's just some residual refrigerant that is causing the drop in vacuum over time. It was initially just a 30 minute evac and check before committing to the full 3 hour evacuation. I hate to go through the whole process only to find out there's a leak and then have to do it all again.

[dn010]

Typically you'd check for leaks by pressurizing the system with nitrogen, not by seeing if it holds a vacuum.

[Big Fins]

I've always used vacuum. But then I'm an old fossil too.

Lot's of variables in vacuum. The grommets in the manifold set hoses. The seats in the manifold valves themselves and of course any connections anywhere in the system. Back then we bought R-12 for $.50/can. Now the greenies have all but outlawed it.

["Cadillac Kid" Greg Surfas 15364]

When under a vacuum the o-rings are pulled into the fittings sealing up any leaks that might be there when the pressure forces them in the opposite direction. Checking for leaks should be done with the system under at least 50psig.
Greg Surfas

[Cadman-iac]

  I've always just used the vacuum as a test to see if the system was sealed, old school as Big Fins said.
I have a couple of bottles of nitrogen that I use to flush systems with, but hadn't thought about using that to pressurize it with.
Gotta see what I need to get it into a sealed system. Probably have the fittings here somewhere.
I just checked the gages this morning after evacuating it yesterday for a few hours, and it's dropped again, so obviously I have more work to do.
I'm still curious about my original question though. When I capped the lines at the compressor, a few hours later I bumped one off and the system had more pressure in it. It wasn't much, but it was there.
Can residual refrigerant cause the pressure to rise, albeit minimally? Maybe just due to a rise in the outside temperature?
I ask because I've seen refrigerant escaping the drained oil from components when measuring how much to put back in.
Anyway, thanks for your responses, it's much appreciated. I'll make use of this information and correct my mistakes.

Rick

["Cadillac Kid" Greg Surfas 15364]

Getting back to basics, how would you find a leak with the system under vacuum?
Greg Surfas

[Cadman-iac]

Getting back to basics, how would you find a leak with the system under vacuum?
Greg Surfas

Lol!! Very good point. In the past I would just look at the seals again to see if I missed something, but yes, it's definitely not the best way to go about it.

At least with pressure on the system you can spray soapy water on your connections to see if it leaks. I have done that with refrigerant in the system, but I don't like using a can to test with as the cost goes up it seems every week. R134 is twice what it used to be just 4 years ago.
I must admit that I hadn't thought of using the nitrogen for this purpose, I had bought it simply to flush with.

Back to my other question, I'm sure you have seen the refrigerant bubbling out of the oil, I liken it to the carbonation in a soda or beer.
You open the container and the pressure escapes, put the lid back on and the pressure builds up again.
Can the refrigerant do basically the same thing if the system is freshly drained and just a quick vacuum is pulled on it? Any remaining refrigerant escaping the oil will add pressure to the system, or in this case, lessen the vacuum until all has escaped the oil.
Just a theory, no hard facts here. Looking for something either way.

Rick

[Big Fins]

Not being a chemical engineer, I plead ignorance to even thinking that the oil would hold refrigerant. I also haven't done a modern A/C system. We always did the repair, pumped it down for 20 minutes or so, then watched it for another 20 minutes. If it held steady, pump it down for a few hours and recharge. Next!!!

I also never thought to consider the O-rings may move. Being as a system is always under a static pressure when idle, checking at a pressurized level would be the way to go.

["Cadillac Kid" Greg Surfas 15364]

Yes refrigerant DOES mix with the oil and remains so until either the temperature of the oil exceeds the boiling temperature of the refrigerant at the pressure the system is at OR the system is held in deep enough vacuum for long enough to lower the boiling temperature of the refrigerant to that of the system (typically 50 degrees)
Greg Surfas

[Cadman-iac]

Thanks Greg,
I didn't know exactly what the parameters were for the oil/refrigerant separation point, and I wasn't sure if that's what was going on, you answered my question, thank you.
Fins, I'm with you, I'm no engineer, but I did catch that refrigerant bubbling out of the oil.
Since I didn't evacuate the system for very long I figured it must be something like that happening.
I've charged the system with nitrogen and I'm just waiting to see what happens now. Hopefully I was just seeing the oil/refrigerant separation causing the vacuum to drop.

Greg,
Does the refrigerant impregnate, or penetrate, (if that's the right term), the oil with R12 like it does with R134, or does it stay separated?
I haven't worked on a system with R12 in it in so long now I don't remember if I ever noticed that or not.

Rick

["Cadillac Kid" Greg Surfas 15364]

The term is "miscible ". Oil and refrigerant become a mixture

[Cadman-iac]

The term is "miscible ". Oil and refrigerant become a mixture

 That's probably something that I'll never remember, but it's good to know there's a term for it.
 Thank you,

 Rick

[Cadman-iac]

Typically you'd check for leaks by pressurizing the system with nitrogen, not by seeing if it holds a vacuum.

This worked great, I found the leak. It took forever, but I found it.
The compressor shaft seal was the culprit. Not sure why, it was new. The old one was still good, it didn't leak, but the O-ring for the front hub was leaking, and since I had to remove the hub, I figured I would replace the shaft seal since it was so easy to do at this point. I should have just left it well enough alone. Murphys law I suppose.

I tried soapy water on all the connections, nothing. I tried unbolting the compressor and turning it face down, still nothing.
I finally used a piece of hose to my ear and held it against the clutch drive hub and I could just barely hear a faint noise.
So I pulled the clutch hub off so I could look directly at the seal and that's when I verified the leak.
When I turned it face down I thought it would leak oil out around the hub, but the felt strip used as a dust/dirt shield was absorbing the oil.
Now I just need to get another seal, and hopefully I'll get it back together tomorrow.
Thank you Dan for the suggestion, and thanks to Greg and Big Fins for your input, I definitely appreciate it.

Rick

[Cadman-iac]

  Update:

 New seal is in and it held the pressure of the nitrogen to 120psi for 4 hours, then evacuated the system and it held vacuum at 29 inches for 3 hours, so I charged it.
  Many thanks again for your expertise and suggestions, it made this job much easier on me.

 Rick