Braking problem

I have a 1990 B2200. I was having a brake drag problem but I replaced the master cyclinder and the drag problem has gone away. However I am still having a braking problem. What happens is that as I am almost at a stop the brake will seem to release. So if I am braking with a constant pressure, just before I stop, maybe at about 5 km/h or so, the brake lets go and I roll easier. If I hit the brake hard when this happens I will stop. I guess the best way to describe it would be like if I was braking with a constant pressure, just before stopping let up on the brakes. This was happening before I replaced the master cylinder, so I don't think that is the problem since it hasn't changed with the new (used from the auto wreckers) master cylinder.
Any ideas would be greatly appreciated.

I had a similar problem with my 90 2200. I first bled the brakes and it seemed to cure the problem. Wouldn't hurt to check the operation of the calipers either. I was doing a brake job and found that the pin that the caliper slides on was frozen with rust and dust. Once my brake job was done I had to bleed the brakes again. No problems since.