You should be able to interrogate the ABS system George, either using the underbonnet box with LED and probe or the"not-quite-OBD" socket in the armrest. Not sure if there were updated optons on the 855/850 for reading the codes but that would be a good starting point.
Rest is "spanner work" but the O/S/F brake and the ABS fault could be linked.
https://www.volvoforums.org.uk/showthread.php?t=83897
That might also be helpful.