99 Sub brake abs problem

I'm having a problem with my 99 4x4 Suburban braking system. The symptons are: during normal or slow braking the abs system engages below about 5mph but not always. I can feel the pedal give way and the abs motor engage.
During hard braking system works normal or OK. I have had the master cylinder replaced with a NAPA reman. unit with no effect, problem still there. I have checked rear wheel cylinders, they are not leaking. Any ideas out there?
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See answer to post below. Also check if the recall affects your truck the symptoms are atypical of the recall issue.

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Depending on where you are located, there may be a recall on your vehicle for this problem. (IOWs, it's a common problem but GM isn't covering it 100%)
If not, the repair is as follows; Raise and support the vehicle. Remove front wheels. Remove front brake calipers and rotors. Remove 2 bolts (each side) that retain the brake splash shield. Remove 1 bolt that retains the wheel speed sensor, remove wheel speed sensor from steering knuckle. Plug wheel speed sensor hole in steering knuckle with a suitably sized plastic plug, the plug needs to fit snugly. Sand blast the area of the knuckle where the wheel speed sensor mounted. Do not allow sand blast sand or other debris to contaminate the wheel bearing. Blow off all debris. Coat the just cleaned area of the steering knuckle with anti-seize or other waterproof brake grease. Clean any rust or debris from the wheel speed sensor. Re-install wheel speed sensor making certain that it is inserted all the way flush with the knuckle. Re-install brake splash shield making certain to torque the bolts to specification, re-install brake rotor, brake caliper and wheel in reverse order of removal. Pump brakes to make certain that all slack is removed from front brakes. Lower vehicle, test drive and verify successful repair.
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thanks to those who responded
I'm located in NE Ohio, so I called the local dealer and he put the VIN no. in the computer and no recall has been issued for this vehicle, but he was aware of the problem and wanted $500 to fix it. So will try to fix it myself, thnx again for the help!

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Paul wrote:

The Front wheel speed sensors (either one or both is bad)..... If you have no brakes below 4 MPH... both are bad.....
Full retail for each is about $125 each...... Plus labor..
This is a GM recall item in salty Winter states & Canada....
If you gripe to 'a Good Dealer', there may be a secret sensor Warranty IF your truck is under 60,000 miles........
This is a corrosion problem that lets the sensor to rotor gap open up so that the ABS computer thinks your truck wheel has locked up, therefore the Computer either pulses or releases your brakes....
Be Real Careful in Gas Stations.... No Brakes !!
And get 'em fixed by GM for Free.... Call their (GM) Hot Line....
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Paul wrote:

By the way... ABS sensor Problem is for 99 'New Body Style & up'
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