1990 Chevy Astro antilock brakes

1990 Chevy Astro mileage: 112,758. My 90 Astro with antilock brakes and hydro-boost during medium to hard braking at high speeds makes a strange noise (sounds like tires on a steel deck bridge). The brake pedal is being forced up against your resistance. If you let up off the pedal, and reapply it will generally pull left or right.I replaced the hydro-boost with a used unit. The problem still persists. No problems with power steering, as I understand they operate off of the same pump. Any suggestions on how to solve this problem?