My cap does have an inner rubber washer, however(m
That doesn't block the 2 vent holes for the cap. The hole are visible on the outside of the cap under where the connector housing meets the side of the cap.
I found that Summit Racing also sells this brake fluid tester for $25.
I should have it in a few days. My fluid is ~2 years old and ready to be changed. I'll check what's in the resevoir and what comes out of the calipers to see if there is any real difference.