Just bleeding my brakes guys and wondering what would be causing this. I bled the master cylinder. No air bubbles. Then went to the right caliper (from a sitting on the bike point of view) and bled that brake. No air bubbles. Then the left brake and getting ever so tiny bubbles in the line. I am using a pneumatic system to bleed. Tried resetting the line on the bleed nipple numerous times and still tiny bubbles. It feel like I get good brake pressure on the brake lever. Can't see any leaks anywhere. Is this normal to get the tiniest of bubbles? Thx!

Possible a too far open nipple, the pneumatic system might be sucking air thru the threads, and this is where you see the bubbles appear?

Thx hub will look into that

Hub is correct.

Is it OK to put little grease around the thread when bleeding brake? Grease prevents air sucked through the thread or will grease contaminate the line?

you should only be using minimal pressure to bleed, I typically don't exceed 3 inches hg. This allows the brake fluid to form a seal in the threads. If you use too much vacuum, you'll pull air through the threads like Hub's saying.

Is it OK to put little grease around the thread when bleeding brake?

Purge is fluid behind the last thread at the bottom of the nipple. So just close the nipple as you keep moving the lever to the grip. Air happens way up and around the threads you see the mix of both.

I always get tiny bubbles even after the system is totally bled. It's just air from OUTSIDE getting sucked past the threads and and going right back out the nipple. No problem as long as the brake feels good and firm.