"For now I was just planning on continuing to apply penetrant, perhaps giving it light taps on the side with a hammer and eventually try to (gently) hammer on the tightest fitting socket I can find and try a 1/4" breaker bar - clearance is pretty tight there. "

If there was one single outstanding important lesson I learned from the past 15 years of Volvo... Impact over brute force. Followed with patience.

Brute force is that breaker bar. You gradually apply the torque and twist the bolt into two pieces. Or your socket doesn't get "hammered on" and you force your next move to be vise grips.

Get a solid wrench on it if possible and deliver the torque sharply with a hammer to break bonds between the threads -- those bonds the penetrant never touched. And remember the brake line nuts will break loose at the threads and remain seized to the line itself. Bleeder valves are hollow and weak. Put your 8mm box wrench on them and tap the end of the wrench with a small hammer. Don't use cheap "HF" line wrenches when the consequences would pay for a decent one.

Heat helps, but in the hydraulics you really have to apply some common sense safety before you get the torch out.

--
Art Benstein near Baltimore

Progression of engineering education: a good planner needs to know at least a little bit about a great variety of topics, and engineers often succeed by specializing, so we planners like to say that over time, planners learn less and less about more and more until eventually they know nothing about everything. Engineers learn more and more about less and less until they eventually know everything about nothing.