Your issue with the o-ring reminds me of the time my buddy and I tried replacing the flex-line to one of his rear brake calipers. We rented one of the vacuum bleeder pumps from Auto-Zone, replaced the flex line, slightly loosened the bleeder cap and went to town. Sucked a ton of air out as the new line filled, but then there was a constant stream of tiny bubbles. He wanted to make absolutely sure that the line was clear, so he kept pumping and kept filling the reservoir. At first we thought maybe something about the way his original flex line failed catastrophically had introduced air all the way through the system, but the bubbles just kept coming. A full bottle of DOT3 later, we were questioning and trying to figure out where the bubbles were coming from, which turned into a fine-comb search of the entire brake line all the way back to the front of the car looking for pinholes, weeping, etc. After maybe 2 hours of searching, we found nothing. Continued pumping through a second bottle of DOT3, and then we realized that the air was seeping in through the threads of the bleeder cap and could be safely ignored. We felt real stupid.

It's about time I do a brake flush on my car, so it's good I remembered this story so it's fresh in my mind.