Some of the symptoms you're describing could also be caused by an issue with the power brake booster. It's powered by engine vacuum, so check the big vacuum hose running from the booster to the engine for signs of tears, cracks or not being securely clamped down at both ends.
It's possible the booster itself has failed, but in over 40 years of working on cars I have yet to replace one. I have, however, replaced and repaired damaged hoses to the booster.