Have you adjusted the controller it may be on a lower setting which would reduce the amount of braking for the 5th wheel even using the manual lever.
Adjusting the brakes on the 5th wheel will make a difference if the brakes were improperly adjusted to begin with. Even with new brakes they should have been properly adjusted and you should be able feel the 5th wheel brakes come on when using the manual lever.
It sounds like either the brakes weren't properly adjusted or the controller is turned to too low of a setting.