Lost vacuum on high pressure side of AC?

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I lost the freon in my AC a couple of weeks ago and have been trying to hunt down the issue ever since. I replaced the expansion valve & o-rings when I saw some dye on them, but that doesnt appear to have fixed the issue.

I haven't been able to find dye ANYWHERE else. When I draw a vacuum I lose it over the course of about 1 hour. BUT, last night I shut off the valve to the high pressure side (low pressure still connected) and this morning the gauge hadn't moved at all.

I dont understand AC well enough to know what elements are on the HP side, but NOT on the LP side.

Anyone have any ideas?
 
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Pull a vacuum on it again. This time shut off both valves (high and low side). See if the pressure holds. You are losing the vacuum though your gauge set, hose or fitting most likely.
Are you saying shut it off at the compressor? I assume so as in a normal test you shut off at the gauges.

Essentially this is what I did (although only on the HP side - LP is still open all the way up to the gauge set) and it appears to be holding vacuum just fine.

I thought there was an airtight seal between HP and LP, otherwise wouldn't it all be the same pressure?
 
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I thought there was an airtight seal between HP and LP, otherwise wouldn't it all be the same pressure?
It's my understanding the the high and low sides should equalize fairly quickly after turning off the AC. I think the guideline I read was that the two sides should begin equalizing within an hour or so. I just redid my entire AC system and the gauges showed that it was visibly equalizing immediately after shutdown. Before, the sides weren't equalizing after an hour, which I chalked up to a bad TXV.
 
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It's my understanding the the high and low sides should equalize fairly quickly after turning off the AC. I think the guideline I read was that the two sides should begin equalizing within an hour or so. I just redid my entire AC system and the gauges showed that it was visibly equalizing immediately after shutdown. Before, the sides weren't equalizing after an hour, which I chalked up to a bad TXV.
Thats super interesting, especially as I just replaced my expansion valve. If thats accurate than holding vacuum on the LP side, but NOT the HP sides means either my expansion valve is bad (unlikely as its brand new) or it is a gauge/fitting issue and external to my system. That would sure be nice, Im really tired of futzing around with this thing and baking in +100* weather.....
 
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No just at the gauges. They don't happen to be Harbor Freight by any chance?
Not HF. They're the rentals from oreilly. They have shut-off valves at the gauges (you shut them off there to seal the system from the vacuum pump) and they have valves at the end of the hose where it attaches to the compressor.

Typically you open both to draw a vacuum, then shut off the valves by the gauge to isolate from the vacuum pump and measure leakage.

Looks like this:

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I think I can shut off both to leave a vacuum on my system and check for leaks in the gauge and hoses. I'll try it tomorrow after work.
 
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I used HF gauges and they worked fine, but the hoses gave me some trouble. One of the rubber seals fell out! I spent an hour looking for a leak in my system before realizing that it might just be a bad seal at the gauges. Inspected them and sure enough one was missing. I used an extra o-ring and that worked well enough to finish the job.
 
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@flintknapper nailed it. After a long series of steps with confusing results I went and got another manifold gauge set from a different autobox store and threw it on there. It held vacuum overnight without losing any, so my conclusion is that the system is airtight.

I'll flush the system, replace the drier and vacuum it down and then do a recharge with new freon. Just in time for the temps to drop and I wont need it :(.....
 

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