This doesn't sound like abs to me. Abs is an add on. The hydraulics are entirely conventional. If they can't make enough decel abs will just sit there idle. Abs faults bring the warning light on. Don't let black box terror ruin your diagnostic process, millions of o-rings fail every year but the internet is full of stories about how people changed the abs ECU three times, couldn't make it work, ripped out the whole hydraulic system and repiped from scratch. It works after that because the o-rings are new!
Sounds like air in the system to me.
Andy