spongy brake pedal
 

I got an 02 quad cab. I recently replaced the power steering pump and had just installed 35s, everything was fine. Then yesterday after helping my friend pull his dead f250 out of a field with 3-4 ft of snow my brakes got really spongy. I looked at fluid level which was good, then checked for leaks. Everything looked good. The brakes will be fine at first but then they go to the floor and feels like it will barely stop. On my way to work today at a stop light i held the brakes and was able to spin the rear tires. Which before what ever way i tried i could not get those tires to even break loose. So it leads me to think either that proportioning valve or something with the rear breaks. What do you guys think? Is their any way to test the hydroboost system?

I was just telling another guy on here I would check the proportioning valve mine leaked out of the needle valve on the top it was a tiny leak and once I pulled the rubber cap off and hit the brakes you could watch it leak