[Mangled03gmc]

Ok so the other day I noticed the brakes on the truck getting a little soft, then the next more, now this morning they would go all the way to the floor, no brakes at all, reguardless if the truck is running or not. I am not versed huge in the whole hydro boost thing, but do know not to jump to the conclusion of a bad master cylinder, even though that is prob the issue. Turing the wheel lock to lock and holding it, i hear no hissing sound at all. Could the spool valve or accumulator be bad? If so what is the best way to check this? No leaks anywhere in the system, as I do not see fluid disappearing. Thanks for any help or info guys..