My 87 F-150 rear antilock light was coming on and off every now and again, but now it is on all the time...Any help w/ what it could be.Thanks

I had the same problem on my '88. It turned out to be a blown wheel cylinder. Once I replaced it and adjusted the brakes, the light went out. I would also check out the porter valve located on the chassis on the driver side rail. Sometimes they get hit by debris when off roading. Just a thought.

Mine lights up when fluid is low,check..

Had same problem on 89, replaced the bad sensor at rear pumpkin and light went out, anti lock sysstem works fine.