brake power loss at low speed

Could be the master cylinder failing but usually this is what happens when the brake pads are worn out as the caliper is trying to stop the vehicle with very little friction material left, this happened on my Durango even though I new it had new pads my wife worn them out in under 10,000 miles

I also experienced this and found that it was the speed sensor.

just had simialr prob on mine had 2 put new bearings in front with sensors was causing the breaks to not work right cost about 650 to fix that included labor