As panic mode said, the motor should be replaced. The brakes in Fanuc motors are inside the front of the motor assembly where any lubricant getting past the front seal can affect the brake rather quickly. If the robot hasn't had the grease exchanged in a while, I would consider doing that while it's down for the motor R&R. If the old grease has thinned out, it's easier to get past the front seal. Brakes also wear out from being engaged too many times while the robot is in motion.
Be careful installing the input gear on the new motor so its bearings are not stressed.
Make sure to use a new o-ring for the motor.
When installing the motor, make sure it seats flat to the casting once you find the sweet spot centered in the 3 gears of the RV reducer. A worn RV & input shaft can sometimes allow enough play for the input shaft to engage only 2 of the 3 gears and essentially lock the RV.