Help with AC

[tdangle]

I rebuilt my AC and charged with R134a. Everything worked good, high side was a little high per the R134 charts, but I understand that is typical of retrofits. Yesterday it was hot. Turned on AC and nothing but hot air. Gauge readings were 31/152. Looks like I lost Freon. I inspected all the connections (new O-Rings everywhere) and don't see any signs of oil which should indicate a leak, I think. How should I proceed to find the leak. (I do have the original compressor which is bone dry around the front)