97 A/C leaking

[70GMCer]

Where is a common area for the ac system to leak?

On my 97 GMC ext cab, I've looked at all the exposed areas under the hood for any oil leakage.

Compressor has been changed recently around 6 months back by the PO.

I've even leaked checked using my air compressor to 125 PSI using a sound leak detector. Nothing found.

I don't have access to Nitrogen to test with.

I've vacuum down the system & let set for around a week, & it held.

Then I have recharged & within a month, it's almost out of Freon again.

I've did this 2 times.

I'm hoping that the evap under the dash is not leaking. I don't want to have to remove the big monster.

I really don't want to use one of those leak sealants unless someone has a preference to using one that does work.

Any ideas ???