I'll take the brushes out again and give it another shot.Looks like the paddle switch in the diagram. Which has a switch disabling key.
Those old brush leads sure look like they got hot. Sometimes new brushes don't seat well.
May want to revisit the brushes. Especially if the commutator got heated up too much.
I just ordered a new circuit board, should get it next week. Let's see if that fixes the problem.