Hello. I have a 98 Ford Expedition. My ABS brake sometimes comes on... When going in a slow turn, my pedal sometimes shakes and does not brake. I usually have to pump the pedal to get it to brake. What is this? Thanks for your advice in advance.

Have a scan done to determine which codes are stored for the ABS system. You could have a sensor that's reading erratically, causing the computer to think the wheel is slipping and kicking on the ABS when it doesn't need to.