How to find leak in 98 Suburban with dual AC Are there any simple leak fixes?

My 1998 K2500 Suburban is in overall great shape. I keep it maintained very well. When the air conditioner R134 charge leaked down, I tried extensively, but could not find the leak. Turned out to only be the schrader valves on the compressor (both high and low pressure lines). Didn't replace the valves. Just re-gassed the system and screwed on red and blue plastic caps which have "O" rings in them. No more problems. That has been about two or three months ago and the A/C works fantastically.