Let's diagnose my suddenly non-functional A/C (BMW E82 content)

The A/C in my 128i has never been what I would call good, especially given that the car is black - it's basically a solar oven. It also has had a very slow leak somewhere for as long as I've owned it - it has needed charging about once a year. Normally, this is indicated by a slow loss of cooling, starting at the driver's side vent. It's a pain, but given that the most likely spot for the leak is the evaporator, and pulling that is a hideous job (the whole dash has to come out), I've been living with it. In fact, I just charged it up a few weeks ago, and all was good.

Then yesterday happened. Hopped in the car in the morning and noticed pretty quickly that there was zero cooling happening. I cycled the system several times but nothing. I checked the pressures this morning (70degF ambient). Static is 68psi, which seems to be correct. Low side is ~12psi, and high side reached as high as ~120psi. This car has a variable displacement compressor rather than a traditional clutched system, so it's hard to interpret the high side pressure definitively, but both of these seem low to my understanding (which is deeply limited).

According to this chart (I can't find one that's BMW-specific, but I don't think it should matter), the most likely culprit seems to be the expansion valve, but I'm not terribly confident in my diagnosis. Comments from anyone who actually knows what they're doing would be most helpful.

Be thankful it appears you are better off that I was with two BMWs of the 2007-2008 vintage -- both developed leaks in the evaporators.  

Pretty much the entire interior is assembled around the evaporator.

In reply to Karacticus :

Oh, there's no guarantee that I'm better off. As noted, I know all too well that that's a possibility, but the current issue seems like something different.

Very low.  High side pressures should be in the high 200s psi.  Low side should be in the 20s/30s when the compressor is on.  They'll all have a slightly different spec, but you're definitely low on refrigerant.

Hard to tell if anything else is wrong, but 125psi isn't enough to force evaporation through the expansion valve, hence no cooling

If it is like the four cylinder X3s, the expansion valve will buzz and vibrate when the system is low.

I have had compressor failures on BMWs where the variable displacement actuator thingus stopped working.  At that point you get no refrigerant motion as it defaults to zero flow.

I'll second some BMWs getting loud when low on refrigerant.  Some of those systems aren't silent anyway.  My E38 A/C is always a little noisy at low RPM, but it'll cool a warehouse.  When I first got the car it cooled fine, but the A/C was noticeably loud.  Put the gauges on it and it was a little low.  Got much quieter after topping it off. 

Pressures are definitely low when running.  I wouldn't expect low side pressures under 25 - 30 on most automotive R134a systems.  I'd be inclined to throw some refrigerant in, confirm pressures behave normally and the system starts cooling, then figure out where the leak is.  That'll at least tell you if you're chasing just a leak or if there's another problem preventing it from cooling properly. 

I'll have to talk to my indy, but the more I think about this, the more I'm inclined to just take it to him and have the whole system gone over. I've had the car for five years, and the A/C has never been what it should be. There's definitely one or more leaks, but I'm not practiced in dealing with A/C systems at all, and I don't trust myself to diagnose it properly. Condenser, evaporator, expansion valve, and drier are under $400 for OEM parts. The labor is the killer, but I don't want to rip it apart myself and screw something up for my lack of knowledge.