One of two things likely happened, you messed up the spur gear installation or the mesh between the pinion on the motor and the spur gear is wrong. The fact you state the motor wires are hot and the “braking” action indicates the mesh is too tight. But it’s still possible you forgot a part or put the spur gear on backwards causing an issue. Let’s stick with mesh for now.

If you are using the little plastic piece with the 3 holes in it to adjust the mesh, check the manual (Available @ Traxxas.com) and make sure you are using the correct hole. Also make sure you have the bottom motor mount screw in the correct position. If you are not using that plastic piece loosen the mesh. When set correctly, hold the pinion from moving and rock the spur gear back and forth. There should be a slight amount of play between the gears, no play is too tight. You can try doing the slipping a piece of paper between the gears method described in the Traxxas manual but that always ends up too tight for me.