Why would the brakes on your 1991 Ford Explorer Eddie Bauer go soft?

A Couple Of Things. Your Brake Shoes Could Be Worn Enough To Need Changed. But It Sounds As If You Have Air In The Brake Lines, Or A Bad Master Cylinder. I Would Suggest You Check The Front Pads To See About Replacing Them, Then Bleed The Front Calipers Also, I Think This Will Take Care Of It. Otherwise A Full Brake Job May Need To Be Done Installing Brake Shoes And Pads On The Front And Rear. Bleed The Front Then Bleed The Rear( Start by spraying a good Penetrating Fluid On All The Bleeder Screws And Allow A Few Hours To Let It Soak In. This Will Help Keep From Breaking Them Off. Also I Never Use WD40 For Anything Like This. Get A Good Brand PB, CRC, Knocker Loose. Just About Anything But WD40. WD40 Is Just A Water Displacement Chemical Anyway ) If You Find You Need Shoes And Pads. Suggestion++++ Install Both Front Break Hoses To The Calipers, You Will Find This To Be Safe And Best To Do. Reason These Hoses Have A Tremendious Amount Of Pressure On Them When You Put The Brakes On. Also Over Time They Rot On The Inside And Restrict Brake Fluid From Flowing From The Caliuper Back To The Master Cylinder Which Will Mess Up Your Caliper And You Will Be Out More Money To Do It Anyway. Gosh This Is A Lot Of Stuff This Should Help