FWIW, I've repaired hundreds of can motor equipped locomotives. It's actually very rare for the can motor brushes to need any maintenance, that's why they call them "maintenance free" motors. I've personally never felt the need to "clean" any can motor brushes or commutators. I've had a few can motors where they've just developed issues in the bronze bushings and get harder to rotate. Occasionally, some oil will free them, but usually there's nothing frees them up and I replace them. Can motor commutators are nothing like the flat commutator of a Pulmore motor, they're a different breed and really don't require the same kind of maintenance you have to do on a Pulmore AC motor.
I have recently purchased both a Norfolk Southern and a Lehigh Valley mth premier heritage diesels. I noticed that they are running sluggish or barely at all. I noticed yellowish grease coming out the bottom of the rear trucks on both engines. Could this be the culprit if it’s gotten hard over time? What’s the best remedy to remove?
I seriously doubt the grease you see has anything to do with the can motor itself, it's almost surely the lube that was applied to the truck gear and motor worm. In extreme cases, I take the trucks off totally, remove any painted parts like sideframes or pilots, and then hit them with carb cleaner. Once I've totally removed the 20 year old grease, I proper lube them with Red-N-Tacky on the worm and internal gear, and my go-to lube, 5W-20W motor oil on all the axles and a drop on the external gears. I use very little lube on external gears as that just invites them to pick up bits of ballast and jam the gears.