brakes do not work when applied and hit bump at same time

It sounds as if the Anti-lock Brake System (ABS) is activating in your 2001 Ford Excursion, it may be activating normally. When a wheel stops for any reason during braking the ABS system will engage. Both cases you describe are situations when a wheel can stop during braking. The normal driver's reaction when the ABS engages is to let off the brake pedal - this is exactly the wrong thing to do! When this happens you should try and keep steady pressure on the brake pedal so you don't lessen the braking force at the wheels still capable of stopping the vehicle. If you think the ABS is activating when it should not that could be the result of a faulty wheel speed sensor. Road testing with a qualified technician may be the best may to confirm normal operation of the ABS system.