Crap/old brake fluid can contain a large amount of moisture, this moisture can attack things that normally would rust like valves in the abs unit, theyu don't normally rust as the presence of brake fluid with no air doesn't support corrosion, add in moisture though, and it's a different ball game, and costs very little to find out.
Moisture in the brakes is bad, and my metre shows a warning at just 1.5% of needing changing within 6 months, anything over 3% is disaster.
ABS is also part of the mot, and I would imagine invalidate insurance cover if removed from a car the insurer expects to have this safety feature.
Can't say 100% it is that, but worth a try ?
tabetha