If your compressor is venting refrigerant, there should be oil all over it.

The dye and blacklight is nice, but it won't show anything if your leak is not in the engine compartment.

Use a sniffer, if you have it. The way to check the evap (the way I use anyway) is to turn the A/C off, and the fan to full for about 10 minutes; this could clear out any moisture that can cause a false reading. After that, turn everything off, close/tape off your vents and wait for about 5 minutes then "sniff" the condensate drain tube. You've got a decent sized leak, so it should be pretty apparent if it's inside.

If you don't have a sniffer, evac the system and charge it with nitrogen. Tell everyone to quit making noise, shut the shop doors and listen for the leak. I probably find 9/10 leaks using nitro. Now, don't laugh, but if the air is relatively still, you can locate the source of the sound (in the engine compartment) by using a stick of incense - just watch the smoke; it'll pick up even the slightest airflow. If it's inside, you should be able to hear it easily with all the doors shut (and the vents unsealed). Again, you have a decent sized leak, so you should be able to hear it.

A bottle of Super Blu is a good thing to keep around for leaks as well. It stays in liquid form from -30degF to 200degF. Spray it on fittings, hoses, etc... It will bubble up (and stay bubbled) on even the smallest of leaks.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.