Ford Repair: brake bleeding, differential valve, bleeders

Question
Hi, We are having trouble with the brakes on a 1992 f-250. We accidentally let air pump into the brake system after we let the fluid run out. We have bled the brakes and the pedal seems spongy. We have a repair book and it says to centralize the pressure differential valve. I don't know what that means.
Thanks

Answer
I have had luck with the following procedure:
Loosen all 4 bleeder screws and fill the master cylinder to the rim.  Allow the brakes to drip and keep the reservoir full.  All 4 brakes should be draining about the same amount.  When you've almost run 1 quart of brake fluid through, start closing off bleeder screws (starting with the fastest-dripping ones) and if there are any slow ones, let them go a few minutes longer.  Do not pump the pedal.  Finally after all the bleeders are closed, make sure the reservoir is full and the cap is on, and pump the brakes.  If there is any sponge left, continue pumping the brakes for a few minutes and most of the time that will clear it up.  If not, the master cylinder has a bad valve and you'll have to replace it to get a good pedal again.  Hope this helps,
Jason