Allow me to start this off with a very brief "About me". I'm brand new to this site, but not new to RC. Started in this hobby about 17 years ago, and just recently left that crapshoot forum at RCUniverse.

Anyways, that being said, it's been about 15 years since I've rebuilt or had any experience with mod motors, specifically with brushes and spring tensions. I do understand that I'll still have to do a break-in procedure.

My question is:

Assuming I have every type of brush and spring tension available, which I do, what would be the best to use for dirty, muddy, sometimes nasty conditions? Grooved? Flat? Copper? Seriously, I've got lots to choose from, from generic junkers to old Trinity P94 and everything in between.

Same for spring tension. Light, medium, heavy, etc etc.

Motors are an old Trinity P2K2 Pro, Midnight Pro 2, and Speed 19

Luckily, I've forgotten more than I ever knew about brushed motors.

Well I went ahead and put in a medium spring and serrated silver brush combo, we'll see how it behaves. I like them for our trail rigs since they are naturally waterproof and we do a lot of mudding and creek crossings

I'd go with a stiff spring to help keep the brush in contact, resist junk getting in, and better for low speed power likely needed for muddy conditions.

Not convinced brushed motors are much more waterproof. Don't both types have epoxy coated windings? Seems getting rid of the brush & commutator would be a big advantage. There may be an advantage with brushed if it's a bushing type, most all brushless have bearings best as I know. Either way I'd be more concerned with the speed control and receiver.