What do i check with the abs system.could it be causing the brakes to lock down?

Have you checked the connectors in the wiring harnesses for the ABS unit and associated components for water intrusion or corrosion?

The codes that you've posted (details below) all point to a poor electrical connection in the circuit(s) or an open or short in the circuit(s). That could be either due to corrosion, liquid intrusion, a wiring harness rubbing against something hot or something that vibrates and has damaged the wiring.

If the wiring and connections are all intact and there are no opens, shorts to power or shorts to ground, the next think to look at would be the modules and/or sensors to make sure they're operating within spec.

U1003 Possible Causes:
*Faulty Anti-lock Brake System (ABS)
*Module Anti-lock Brake System (ABS)
*Module harness is open or shorted Anti-lock Brake System (ABS)
*Module circuit poor electrical connection

The Diagnostic Trouble Code (DTC) sets if the Anti-lock Brake System (ABS) Module fails to receive bus messages from the Dynamic Sensor for approximately 150 ms.

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C0020-1c Possible Causes:
*Faulty Anti-Lock Brake System (ABS) *Control Module Faulty Hydraulic Control Unit (HCU)
*Hydraulic Control Unit (HCU) harness is open or shorted Hydraulic Control Unit
*(HCU) circuit poor electrical connection

The Hydraulic Control Unit (HCU) consists of a valve body, pump motor, and inlet/outlets for the brake lines. The combined HCU and Antilock Brake System (ABS) module is located forward of the master cylinder.

The Anti-Lock Brake System (ABS) Module monitors the ABS Pump Motor. The ABS Module sets the OBDII code when the ABS Pump Motor is not to factory specifications.

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U0125 Possible Causes:
*Faulty Dynamics Sensor Dynamics Sensor
*Harness is open or shorted Dynamics
*Sensor circuit poor electrical connection

The Diagnostic Trouble Code (DTC) sets if the Anti-Lock Brakes System (ABS) Module fails to receive bus messages from the Dynamics Sensor.

ABS system has nothing to do with base brake operation . ABS light on ? There are DTC'S - diagnostic trouble codes ,have codes checked ! No it's not a ground issue .

P0446 Gmc Evaporative Emission Vent System Performance

You have water in the brake system, it boils and applies the brakes.
Different size tires will set a trouble code, but not apply brakes.

The ABS system is an Automatic brake system control! There could be a bad ABS sensor at one of the wheels, or a broken wire to the sensor! The system is computer operated, to sense when the brake pedal is depressed, and causes the brakes to surge, rather than lock up! It is a good idea to have an ASE mechanic check this out! When there is a problem with the brake system, it can cause the car to die when the pedal is depressed to hard or fast!

Your ABS is in problem event not function. Please check it. The ABS will prevent the brake from locked. You can fill it by testing your brake.

ANTI LOCK BRAKE LIGHT.MEANS ANTI LOCK BRAKES NOT WORKING.CHECK ABS FUSE AND RELAY.IF ALL IS GOOD.HAVE THE ABS BRAKE SYSTEM CODE SCAN.NOT SAFE TO KEEP DRIVING.HAVE FIX QUICK AS POSSIBLE.BECAUSE THE ABS BRAKE UNIT IS PART OF YOUR MAIN BRAKING SYSTEM.IF ABS BRAKE UNIT BAD BY BEING PART OF THE MASTERCYLINDER IT CAN CAUSE COMPLETE BRAKE FAILURE.

checked out replace pads /and cut rotors if pads are glazed over [film u were told ] the pads are smoothed over from getting to hot -riding brake or stop and go traffic lot of down hill braking which ever was the cause ur brakes got over heated and glazed over and are very smooth also the heat transferred to the rotors also warped them when u get steering shake when braking 90 percent of the time its warpage in rotor and the only way is to have them removed and put on a cutter to check for out of round also abs doesnt cause this abs [anti-lock braking] is designed to transmit a pulse to ur brakes when it senses a wheel is about to lock up or skid u would feel the pulse in ur brake pedal not ur steering

Hi:
The first thing to determine on your car is if it is safe to drive: Check your brake fluid level in the reservoir, and make sure you don't detect any leakage around any wheel/ tire areas. Your ABS light can be scanned and tell you what tripped it. Most auto parts stores, such as auto zone, will do this for free. The most common cause is usually a bad or dirty wheel speed sensor, but have your car checked out to narrow down the problem.

Check condition of brake drum and brake linings.
Check brake drum not out of round,
Check for fluid or oil leaks in the rear brake area.
Check rear brake proportioning valve is working correctly.

depending on the system if it is 2 channel or 4 channel one of the sensors that are located on the backing plate located behind calipers is eithier faulty or dirty you can try brake cleaner these sensors use a small magnet pickup device to sense when the wheel "locks up"