There's a relay to check on the passenger side firewall behind the hood strut. It powers the abs pump. If the pump doesn't run the accumulator doesn't pressurize and you have a hard pedal. I had the relay fail on my 92. On my 90 this happened and I changed the relay and it didn't help, so then I ordered a new pressure switch. This mounts to the pump motor under the resovoir. It's a pain to change. I had my friend do it for me since it looks like it needs a special socket to fit it (like an oil press switch). Anyhow, it monitors pressure in the brake system and then cycles the pump on and off to pressurize the accumulator as needed. Mine worked after changing the switch.
The pump itself could be bad as well. You might also want to look into replacing the accumulator ball itself ($100 from priorreman.com) because usually it's no good and it cycles the pump too much and the pump burns out. If the pump itself is shot, you are in for a big bill, it'd be easier and cheaper to convert to manual non abs brakes. There's a lot of info on this site for doing the swap. I plan to do it next summer. Search for manual brakes and you should find some info.
I ordered a switch for a buick grand national since they ran this same ~~~~~~ brake setup. It was cheaper than Ford. I still paid about $85 for the switch plus shipping. Ford's was like $115 I think. For less than that you can go buy the stuff new at Auto Zone to swap to non abs. Just need a booster from an 89 base tbird, master and resovoir from a 93 crown vic NOT police package. Some have had to modify the brake rod to push the master cylinder in more. But it can be done in an afternoon in your driveway if you know how to do brake work and then make sure to bleed it out.
If you just change the switch or accumulator you don't need to bleed the brakes.