The over-pressure valve is located on the 3rd stage head and is very simple in construction.
The problem is that this valve pops and briefly blows off "over-pressure" anywhere from zero to a couple of dozen times as the 80 gal. tank is brought up to pressure. There's no consistent pattern.
This should happen only if the 3rd-stage outlet valve to the hose and tank were jammed. I can't find any evidence of that. I don't see any evidence that the blow-off valve is defective, either.
What I'd really like is a complete manual for the compressor or, failing that, an exploded parts diagram. But Atlas-Copco apparently has no record of ever having made this compressor. And gee, it's probably no older than I am.