2000 GMC Sierra

My 2000 GMC Sierra 2500 2whell drive with all disc brakes keeps locking up all four brakes. All components but the hard tubing have been replaced, including hoses. The front wheel bearing assembly had to be replaced with a newer model that has a different sensor, and after that the brakes started locking. Can the two different sensors cause an all wheel lockup? I also read somewhere that a bad spacer in the hydraulic assist assembly might cause the return brake fluid port in the master cylinder to be blocked. Short of taking the thing into a brake repair shop I am at a loss as to what is causing this.
December 18, 2012.

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