The 928 does not have an advanced diagnostic test built in into the ABS unit. It is a pretty reliable system but still sometimes problems exist. Even thou you can see some ABS related errors in the PSD and RDK ecu (later 928 models) the ABS itself became diag capable in the Carrera C4 PDAS units.

Bosch developed a special ABS tester KDAS-0003 which is shown here: