ABS system acting really weird, please help

[ScottN3N]

The ABS brakes on my 94 GC are going goofy. Frequently the ABS will activate at low speed when they should not (dry pavement, etc.) and if you keep your foot on the pedal, it will go to the floor. This problem is intermittent, but somewhat frequent. The car will slow & stop fine until it gets to slow speed. All pads & shoes are new. It has full time 4wd, V-8, front disk & rear drum, 130K miles, brake light on. I suspect that I need a new master cylinder based on experience with other cars, however, all of that extra plumbing for the ABS has me concerned that the ABS unit could be defective and bypassing fluid. Does anybody have similar experience, or any ideas on common problems with this, please? Any ways to troubleshoot this to figure if its the MC or ABS? Thanks in advance for your time and any suggestions.

[fredjacksonsan]

After replacing the master cylinder, if you still experience the problem, check the speed sensors. They indicate to the ABS computer the speed of each wheel. I think on yours the rears are in tandem, that is one sensor checks for both wheels on the rear axle.

Also, make sure that your tires are the same size/make and correct pressure, as with the full time system (and ABS) you could have problems with different sized tires.