It's Possible that your Brake Fluid needs replaced. The Brake Fluid should be replace about every two years.
Hope This Helps.
a soft pedal could be the result of worn down linings, like the rear brakes needing adjustment, leaking hydraulic cylinders....
Easy way... Pull the fuse, drive for a hour or more.. Watch tho... When the ABS is "unplugged" and you first start to go the brakes will very "light" until the vehicle reaches 5mph and the ABS warning light comes on... After that they will work like normal non-abs brakes... This works and pretty much any Ford any year and works good for resetting the brakes after changing the pads and the brakes feel a bit "soft"
If the brake pedal is soft on a 1997 Ford F-150, then there is a good chance that there is air in the brake line. This can be remedied by bleeding the brakes system.
Possibly air in the lines, find someone knowledgable to help you bleed them
Soft brakes are caused by moisture or air in the brake lines. This reduces the pressure in side the system and greatly reduces its stopping power.
If you break pedal is pulsing but is not soft or spongy, that sounds like the rotors on your truck are warped and they need to be turned. My suggestion would be take your truck to a local brake shop and have them take a look.
Probably air in the system.
Spongy or soft brake pedal usually means there is air trapped in the system. Bleed the brakes starting at the RR then LR > RF and ending at the LF.
Here is one possibility. My Ranger is doing the same thing. http://autorepair.about.com/library/faqs/bl714e.htm
Check into the ABS pump or module ---- I would say the pump
One reason could be that air is trapped try bleeding them.
Air trapped in the system. Bleed the brakes.