I'm not sure what's happening with your feeling of brakes on one wheel, or on the resevior level changes.
Our brake master is very different from normal brake masters. There isn't a front and back. It's FL, FR, RR, RL. All ports are under the same pressure by the main ABS pump. Note that the ABS pump is what also provides the power assist.
I'd start with a brake inspection all the way around. you could have multiple issues. And blowing hard brake line makes me suspcious of some sort of contamination in the hydraulic system. I suppose it's possible you have a master cylinder failure in the brakes. You might also have a bad caliper on one side up front giving you the feeling that it is grabbing better than others.
Unless you can find something obvious, I'd start with flush the whole brake system with fresh, clean brake fluid. Replace any obviously bad brake components, and then see how it performs.
If the pedal isn't firm right away I'd be thinking a problem with the master cylinder somehow jammed into the bore, or the bore of the cylinder is damaged. But these are all guesses using basic logic. I've no experience with such a problem.