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I had the same thing happen. Is your brake light on.(the one that would light up if you have the e brake on). I hooked my pump up to a gauge and got under 10 psi which is weak. so i replaced it and it was good as new. I would test the psi before replacing it. I got mine at the parts store for about 100 bucks.
I am willing to bet it is the pump, but just check to be sure if you have a gauge handy. It is pretty east to change. Just 3 10mm bolts and the belt, but u may have to change pulleys when u get it. It is the same pump as an old 7.3 or 6.9L with the old belt style.