ABS steel pins are guide pins inside the ABS brake system. In the hydraulic block, these pins align the internal solenoid armatures. They ensure that the valves slide straight when the computer activates them. In some caliper setups, they act as guides for the brake pads. They must be straight and rust-free.
If these pins bend or rust, the solenoids or calipers will stick. This prevents the ABS from releasing brake pressure properly. During brake service, cleaning and lubricating all guide pins ensures smooth operation.