Truck drifts to the left when braking?

I have a 1992 GMC 1500 pickup, just got new wheels and tires on it. The old ones were oversized, so I went back to the original size. The old setup felt fine when I drove it, but it was kind of sluggish. With the new smaller tires, the truck seems a little more powerful, but when I brake really hard, it pulls to the left pretty bad. Whats the cause of this?