The function of the ABS system is to prevent brake lock-up by pulsing the hydraulic pressure to the brake cylinders. When ABS fails, the brakes don't pulse. When ABS fails, you have the same braking that cars and trucks had before ABS. About the only time that brakes would lock-up on a heavy motorhome would be on ice or heavy rain. The noise that you hear, when the ABS light is off is the brakes pulsing. The most common cause of ABS failure is a faulty speed sensor on one or more wheels. The ABS compares the speed of all the wheels and pulses the wheels that are out of sync with the others. Sensor failure can be from dirt, open wire or a defective sensor. The front wheels do most of the braking, because inertia shifts forward when you apply the brakes. Therefore the front brakes are most apt to lock-up or be pulsed. You should get a RED brake indicator (like when the emergency brake is on) if you have a major brake failure.
Hope this helps.
