1999 Ford Escort ABS

When at a low rate of speed 10mph or so the abs system responds great, but when you exceed 10mph the break pedal is stiff almost as if it was pushin back, with a vibration when trying to be depressed, changed master cylinder but didn't help, both calipers appear to be fine with equal wear on pads, plenty of fluid when bleeding the system. The pedal pushes back then at a point will drop and vehicle will pull to passenger side as if that was the only brake workin. Confused on why it seems fine at low rate of speed but goes in a loop at higher rate.