No pressure in the crankcase, no blow by, not rings...But remember most crankcases are vented into the first stage, as there will always be SOME blow by, which must be vented. Usually fed into the first stage generally before the first stage valves to lubricate them. Every compressor I have owned is vented in this way, of course I cannot speak for all of them.
Proper test...
Get a dwyer flowmeter from ebay, I can find the blow by spec for you with some hunting....but I would expect less then 10% of compressor output.
Quick and dirty way to check blow by... Somebody here gifted me this trick.
It is late and I am in stupid mode, so I will do the best I can to explain...
Tap into the crankcase anywhere you can. My coltri compressor has a flare fitting just for this...I tapped the oil fill cap on a Bauer for a fitting...
Take a large bottled water jug, fill it with water, submerge it in a larger bucket of water open end down. running compressor...take a hose from the crankcase tap you found....shove it up into the inverted water bottle. as the air is pumped in, it will push the water out, to make this a bit more accurate RAISE the water bottle keep the water level in the bottle JUST a bit below the water level in the bucket, you will have to raise it as the air is pumped in ... time how long it takes to empty the 20 liter water bottle..you will have a very close approximation of the blow by of the compressor. Here large water jugs are 20 liters, the 210 LPM pump I was testing pumped the water out in about 15 seconds...Roughly 1.15 lps blow by x 60 seconds per minute..the compressor was blowing about 70 lpm past the rings, 1/3rd of its output and it was about that slow pumping. This actually works. I hope this was clear enough.