HI John.
No simple test but my bet is on the rear OS sensor or bearing, having rust on the sensor/& or the bearing, wheel off, caliper off, drum disc off and check.
Before doing the above you can ask to use the clubs ABS pulse sensor link below, plug it in and turn the wheel to check out the increments
or a T4 session to cut out the guess work.
https://the75andztclub.co.uk/forum/s...d.php?t=260420