Diagnosing AC issues, E90 edition. Low side builds pressure

I bought my 328xi just about a year ago knowing that the prior owner was having a "slow leak" in the ac.  It worked fine the remainder of summer 2023.  The car sat parked all winter.  When I started driving it again, the ac had no freon.  I recharged it with stuff from AutoZone.  It worked fine for a month or so.  It then stopped working.  I plugged in the gauge that came with my recharge can to the low pressure port.  The pressure was in the red zone, I forget the exact number.  I released some pressure (yeah, yeah, I know...) and the ac worked great again, for a few weeks.  Same thing, pressure is high.  Released again with same results, working ac for a few weeks.  Now it's dead again, with pressure high again.  What could be causing this?
 

Are you checking pressure with engine running and system ON?

AngryCorvair (Forum Supporter) said:

Are you checking pressure with engine running and system ON?

Yes, car was running with AC set on MAX.  This time the reading was fluctuating some, but still high.

"No freon"-- none at all?  Did you pull a vacuum before recharging it?  Maybe there was some air in the system?

Without knowing the high side pressure, it's hard to say, but make sure the engine fan is working and the condenser isn't blocked, impeding airflow (these would cause low high side pressure). Could also be a bad expansion valve (this would show normal high side pressure with a proper charge).

Sadly, the slow leak is likely to be the evaporator - ask me how I know. It's not a small job to replace. Try replacing the Schraeder valves first, as those are cheap and do leak. OE valves and caps (replace those too, or at least the O-rings) are cheap.

From my research you really need a gauge set so you can see what is going on with the high side.  I bought my DD 2 years ago with non working AC and I finally got around to fixing it over Fathers Day weekend.  With my gauge set hooked up the low side was higher than it should be but the high side didn't move at all.  After poking around on the internet it seemed the most likely reason was a non working compressor.  I ordered an AC kit from RA and replaced the compressor, condenser and expansion valve.  Once I got the old stuff out the original compressor was crunchy when you turned it, my guess was the clutch had failed.  After putting it all back together it now has ice cold AC and starts cooling very quickly.

I would rent a gauge set from the parts store and go from there, you can also get a vacuum pump for when you are ready to recharge the system.

Did you charge it with pressure gauges on both the high and low sides?

Did you use a temperature/pressure chart when you charged it? 

If the answer to either of those questions is no, then you probably overcharged it. 

An overcharged system will work when it's not working too hard. As ambient temps climb, pressures in the system climb. That changes where in the system pressure drops. If that happens outside the evaporator coil you don't get any cooling. 

Edit: This is the chart I usually use.