Old news. Mr.5 did a DIY fix on this. There is a rubber grommet in that attaches the brake booster to one of it's vacuum lines. It is in a VERY tricky position. When the grommet stops sealing correctly around the vacuum line, you get a slow vacuum leak. Overnight, your brake pedal becomes hard. Ultimately, if this is left unchecked for a very long period of time, it will cause vacuum issues at idle and during deceleration.
The solution is very cheap: $5.00 for a new gromet and a case of beer for the guy with a set of midget hands to have it replaced. I sure as hell can't fit my hands in that spot.
EDIT: It was not Mr.5 after all: