booster is an option on these units, I have very few in the field with a booster.
If it is boosted that would be the obvious place to start.
if not, I would first ensure the diff oil is the correct oil, it should be a mobil 424 or equivalent, this oil floods the brakes.
I would be curious if the dealer removed and inspected the pistons for the brakes?
Its either the pistons are not moving freely or the master cylinder, ensure the master cylinder oil is 32 W hydraulic oil also, Some folks like to replace it with dot 3, that may be your problem too