In my opinion you have another issue, not bump steer. Are your front shocks in good shape? You mentioned "Increased the pressure on the steering box." How and why did you do that?

This link provides information about "Bump Steer"
http://en.wikipedia.org/wiki/Bump_steer

I looked at Classic Performance Products website, is this the kit you installed?
http://www.classicperform.com/tech_articles/Power-Steering-Trucks/Power-Steering-Trucks.htm