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[M.L.S.C.]

First off, what do you mean by "fading pedal" - you gradually loose brake assist to it (you have to step on it progressively harder to get truck to stop), or does it try to slowly sink to the floor?

See if I get this straight - you pulled the hose off the vacuum booster, you plugged the hose with your finger, and funky sound disappeared and red "BRAKE" light went off? If so, then the problem ain't in your master cylinder, but the vacuum booster tis mounted on, and that the hose attaches to.

How do the brakes feel now, with the hose disconnected - with the truck off and parked somewhere step good on the pedal, it shouldn't fade at all, well maybe just a tiny bit, but that's it!