1993 Mercury Villager Brakes Locking while driving

I am interested in purchasing this vehicle from someone on Craigslist. I am wondering what could cause the brakes to lock up. I am including a copy of the post on Craigslist to find out.

"
1993 Mercury Villager (same as the Nissan Quest) with 173,841 miles. This has been a great work van; it starts up every time and has a good transmission. I used it for short trips like moving furniture but really it�s a mechanics special because it has a few issues that I believe a good mechanic can fix. For Starters the brakes mysteriously lock up for no reason. I�ve had much of the brake system replaced (pads, rotors, brake booster, brake lines, ) but it�s still a mystery. Other issues:
power steering fluid leaks b/c of a bad hose
Radio doesn�t work but I�ve purchased another but haven�t installed
Air conditioning doesn�t work
Front passenger seat is ripped pretty badly

I"ve replaced the CV joints, muffler, and put 2 new tires on it. It also passes emmisions with no problem. This van has leather captains chairs, sunroof, and alloy wheels. It�s not the nicest van or cosmetically perfect but has a lot of life left in her."