The AC on my 2004 Subaru Outback Impreza as weak, so I picked up a 134a recharge bottle with gauge. The compressor turned on and stayed on and I checked the pressure, which read low. I shook the can and added refrigerant, though I briefly did this upside down accidentally. The pressure in the system did not appear to be going up, even after about 1 minute of charging (checked every 10-15 seconds), and now the 28oz bottle is pretty low. The compressor then shut off, the belt squealed, and the pressure reads very high, which I guess is normal if it's off. But now the compressor only comes on for a second or two before shutting back off, and the AC now blows completely warm air. So it's apparently broken. There was also a lot of water coming from under the car (like 2-3 oz or something). Perhaps the gauge just failed, and I overcharged it?
The pressure will look very high if the compressor isn't running.
There are two things that stop the Subaru AC system (three, if you include a serious problem like the compressor breaking).
The first is low pressure. I think you're OK there - because the compressor is coming on for a short time. That also seems (at first glance) to indicate that the actual compressor is OK.
The other issue that Subaru's are susceptible to is AC Clutch wear. On the front of the compressor is the drive belt pulley. Removing this (you probably need a small puller), you'll find a couple of very thin spacers (look like washers). Remove one those, and replace the pulley. This reduces the distance the AC clutch has to travel, and can often solve this issue.
If you've never done this before, it's about a one hour job, and can be done on the driveway without removing anything except the serpentine belt and the clutch pulley. Don't even need to jack the car up (because it's all on the top of the engine)
For more detailed information, google "subaru ac clutch", and look for the posts on how to adjust it. All Subaru models of similar years use the same AC compressor.