I haven't dug into the manual yet and wanted to start here first.

1990 F-150, 4.9, 175,000 mi, E4OD.
Both brake and ABS lights came on after I was lightly spraying the dust off the engine and engine compartment last week. The brake light gets brighter when I apply the emergency brake. They went off the day after I sprayed the engine, but came on the following day and have been on since. I don't have any braking issues and fluid is at level.

 

Check the wiring at the master cylinder. If the fluid level is full, there's probably a bad connection causing both lights to come on.

 

Disconnected and reconnected a few times with no change. Any more suggestions?

 

Make sure the pins haven't started coming loose from the plastic connector. That was what happened on my 87. The plastic plug was secure in the master cylinder, but the wires weren't connected. Other than that, I'm not quite sure what it could be.

 

Did you check the fuse? I know that it is simple, but I just checked the fuses in my 90 F250 and there was NONE in the ABS slot. I put in a fuse and the light turned off.

 

make sure your brake lights work, if not, this could also be the reason. Then you need to start chasing down wires from there.

 

when i first started driving my truck the abs light has been on. it suddenly went off and the brake light came on and a few weeks later (today) the abs light came back on and now there both on. I didnt wash anything down. so I dont know if the lights mean its an old ford or if I actually have a problem but it stops okay.