Thanks again for responding to my problem. I did some more analyzing. I just noticed I had a typo on the year, its a 1998 F-150 with RABS. Where I live its very dry and the temp. is around 80F. I don't think its the rear brake pads absorbing moisture.
My ABS light stays constantly on. My brake pedal feels mushy. When I brake, I have to gently start depressing the brake. And just a little. If I don't the rear locks up immediatly. It is extremely sensitive.
When I come to a complete stop, the brake just a little depressed. At that point I can depress it more and I hear a wooshing sound (and mushy feeling).
Under the master cylinder, if I'm not mistaken its the proportioning valve (the rear brake line goes through this aluminum box and exits to continue to the rear). There is a wire plugged into this box. I disconnected it to see if I feel any change but it stayed the same. I think may because the ABS isn't working and so the rear locks up. I think it is either that valve or the module is defective. Sending the wrong pressure to rear.

Also if I by pass that prop.valve and hook up the line directly to the master cylinder (for the meanwhile until I get the problem resolved), will it work as a normal brake without ABS. Or will I still have the slam-in-the-steeringwheel rear lock up?

I appreciate any suggestion you can give with this problem.