Howdy all, seeking input on a funky brake issue. It's def a safety issue I want to take care of this winter.

First the car is a 67 bird I completely redid the brakes except the pedal in 2005, it may have 10k on it. SSBC front disc conv, SSBC lines, rear rebuilt, new booster, mast cyl and Classic Industries preset brake proportioning valve, everything went smooth and it has been flawless and awesome since.

This summer I had 2 instances where some clown pulled out of a driveway and I had to hammer the brakes and come to a stop from @40mph, manual trans, clutch depressed, when I went to resume travel it was like the brakes didn't release or release completely, I had to hammer the pedal and then things returned to normal.

My guess is the proportioning valve or master cyl??

Any thoughts, I don't to just change parts hoping, thanks.