I have had the same problem, and after 3 months of trying to figure it out,we finally disconnected the master cylinder from the brake booster. The front brakes that we're locking up instantly released. I have searched high and low on the internet for a solution to this problem and I am pretty sure this is finally it. if the new booster we are about to put in does not fix it I will post again. I have replaced the ABS modulator valve, brake hoses,, calipers, pads, master cylinder, and rusty brake lines. Wish I had found this forum months ago...