Jim,
Those motors only have two brushes. They are just simple basic motors. The only bad thing about them is the motor body end caps can be broken around the screw holes by over tightening.
Check the wiring thoroughly. Then check the foot controller and it's wiring.
If all that is good you can remove the brushes from the outside for cleaning and inspection. If they are good then there is two flat pieces of plated steel on the inside that contacts the brush holders and those can become dirty, corroded and even bent. To work on those you'll have to take the motor apart.
I've only had one of those types of motors apart for rewiring and they are simple. They just look different.
Joe