Should my A/C inlet port be bubbling at all?
 

I found this spring that my A/C (R12) was not blowing cold air at all and assumed that it needed a charge. When I unscrewed the cap and inspected the inlet valve at the front of the engine (W123 84 300D Turbo) I saw some moisture and a few tiny bubbles. So I assumed that I had a small leak in that valve.

I went to have it serviced, as I am not set up with any A/C equipment, and pointed out to the mechanic that I noticed the valve may have a leak. He recharged it, it is blowing ice cold now, but upon later inspection, when I unscrewed the cap, I heard a brief hiss from some built up pressure and saw moisture and small bubbles. I went back to the mechanic and he tightened the valve a little bit, and said I should leave the cap on tight as this keeps it from leaking. I am skeptical. I thought the cap was to keep the port clean, not to act as a seal. Can someone confirm this for me?

there should be a NEW schrader valve under the cap if it was serviced by a competent mechanic.
I have the tools to do it under pressure, he should too...

Yes the cap is part of the sealing as the mechanic said. Unlike a tire valve stem, most A/C caps have small o rings or a tapered thread to help seal. I agree with VSTECH. Should have had a new valve as well.