If you're accepting guesses, I suspect the master has an internal leak and is by-passing. Master cylinders can have either internal or external leaks. Normally, this gets worse with time and use and could get you a brake lever coming back to the bar at a bad time.
Cure is to kit it if you must, but replacement is better. If you decide to kit, take the master in to have it rebuilt on the bench. They have the experience and tiny hones to do it right.
You also might have an errant air bubble running amok in your system. I like to tap on the calipers, wiggle the lines, and tap on any T's or junctions, and turn the handlebars left and right. Use a 10mm wrench.
Then bleed the system from the top down starting at the banjo fitting at the master. Crack it under pressure, tighten, then move to the next T or junction down to the calipers. It's a mess, but can help get a stubborn bubble out.
I think unscrewing the cap had nothing to do with it.
