Durango Brake Problem

From : ed

Q: i have a 4x4 99 durango with 75000 miles. recently during hard but not panic braking the brakes engage for a few seconds. then i completely lose braking for a second or two. i brake much harder and i feel the brakes pulsating abs. and finally the brakes engage again. this happens every time i brake moderately hard. it does not happen during normal braking. is the master cylinder the problem or is it an abs problem i dont think its an abs problem because im not panic braking when the problem first occurs. i did not find any brake fluid leakage. the pads were changed 2 years ago and i adjusted the rear shoes at the time. how can i test to see if the master cylinder is causing this problem thanks for any help. ed .