driver's side caliper not working good

I have a E-150 van 351 engine, 1995 188,000 miles, I replace caliper on driver's side a couple of times, i bled the system, which by the way is ABS in four wheels and does not work since I bought it, well the van keeps on pulling to the left when brake applied, i would like to know if there is a certain procedure to bleed the system (ABS) or if there is any other thing I have to check to make it work properly. This is the first time I use this site so I thank you in advance for having a very helpful forum!