Need GMC Van Air Conditioning advice on refrigerant leak

I'm a mechanic but never spent much time on air con repairs. I recently purchased a 1997 GMC Safari but the compressor wouldn't engage so I figured the 134 was low. I had it charged and it worked great for a day but there must be a leak. The condenser looked pretty rough so I bought
an excellent one from a low mileage wreck. When I disconnected the condenser, there was still pressure in the system, not a lot but I would think if there was a leak, there would be no pressure.
I did try a can of red dye but I can't see any obvious leaks. I don't want to start throwing parts at this van. Any air con techs out there have any ideas or the best way to get some cold air from this puppy.

Thanks

Thanks for the email. On possibility is a shaft seal on the compressor. This is a very common failure. The red dye can be very frustrating to use and may not really help at all as you discovered. Refrigerant leak detectors are very accurate and will save you a lot of time by locating a freon leak quickly. There are several manufacturers out there. Robinair makes a really good and simple to use refrigerant leak detector. It works by creating a high voltage corona in the sensing tip. When this field is interrupted by refrigerant, the alarm is triggered. All corona discharge leak detectors are not the same, however -- the quality of key components is critical in providing an effective instrument.