A/C leak after vacuum

no visible sign to leak after doing vacuum, spray test. my question is, what is most common on these trucks to fail first? Vacuum pulls down to 28 lbs then takes about an hour to move 3 to 4 lbs. The only thing I can come up with as quick next check is the o-rings on back of compressor at hose connection before throwing parts at it. System was great and no leaks then all of a sudden lost all refrigerant. I would much rather throw parts at it then pay for diagnostic fees at a shop which I could apply towards cost of parts.

Any help would be great and love your videos, very descriptive and thorough for the DIY guy.