Red,
If that Amber ABS light is Lit then the 4WABS System is OFF; system is designed so that any Sys, failure (any front wheel sensor or VSS (also called ABS or DSS), hydr control unit or controller, etc reverts braking to old school braking without any abs functions.
"...There are three main causes of "brake shudder," which is generally felt as a brake pedal or steering wheel pulsation or vibration. Having verified that your rotors are not warped (i.e. warped rotors are another, more obvious, cause of the brake shudder phenomenon), the following are possible, commonly overlooked, explanations for an experienced brake shudder:
An improperly seated or mounted rotors (i.e. the rotor does not turn perfectly parallel to the brake pad surfaces). This can be due to:
(i) debris trapped between the backside mating surface of the rotor and the wheel hub
(ii) improperly tightened lug nuts/bolts
(iii) pads that are not properly re-aligning themselves after releasing the brake pedal,
due to a very rough or dry (i.e. non-lubricated) brake pad to mounting clip interface
Heavy friction material film transfers on the rotors. Very heavy use of the brake pads, especially use which exceeds the manufacturer's recommended heat range, can result in uneven friction material deposits on the rotors. This results in a "grab-release" effect when the brakes are applied. While this will generally even itself out over time, in extreme cases, the rotors may need to be resurfaced/turned/cut.
Worn suspension parts (i.e. springs, shocks/dampers, bushings, etc.). "Old timers" call this type of suspension shift under torque load "tramp." In this situation, the car's alignment is slightly changing due to brake torque upon brake pedal application. Severe cases of this result in a very notable pull in the steering, and can become evident as a shudder. If this is the cause of your brake shudder, the suspect suspension components should be replaced..."
by Carbotech Engineering