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Some easy things to check. Check that the brake fluid is good. check that the electrical connections at the master cylinder are good and check that both rear brake lights are working.
everything is fine, both tail lights, full fluid, all contectors are there. my master cylinder is slowly leaking between the booster an master cylinder.
the most frequent problem is the sender on top the rear axle the wiring unclips and the old one unbolts look for it on top of the rear axle near the yoke for the driveshaft !
The leaking master cyl is unlikely to be the cause of the ABS light but you should fix it as the brake fluid will leak into the booster and destroy it.
Possibly your next step should be to look for the diagnostic connector and pull the codes. It should be behind the glovebox, a single wire with a colored stripe either loose with a connector on it or plugged into a black wire. If you ground it momenterily with the ign on the ABS light should start to flash codes. The codes will be either a single digit, double digit or a combination of both.
First (as suggested earlier) replace the rear ABS speed sensor on top of the rear differential. This is about a $30 part from Ford and takes about 5 minutes to replace.
If this doen't take care of it, do you notice if your rear brakes drag? If they do when the rear ABS light is on, then you may have issues with the rear ABS valve (located on frame rail directly under steering wheel. I've tried cleaning these but no luck (there is a valve position sensor inside the valve also). You pretty much need to replace it. Unfortunately from Ford, it's a $500 part (no idea why it is so flipping expensive). I used an aftermarket one ($250).