First, the symptoms... When running, pressing the brake in, the pedal slowly sinks to the floor...

Well, a search turned up https://www.rx7club.com/forum/showth...ht=brake+pedal (second post) which leads me to believe that my MBC is to blame.

However, it does NOT seem to sink to the floor, or even at all, when the car is off.. I just wanted to make sure that it's still my MBC to blame before I go get a new one..

-Tesla

 

slowly dropping to the floor can also be related to the a leak in the booster.

 

I couldn't find an obvous leak in the vacuum line to the booster...is there anywhere else it could be? Any easy way to determine which (booster or MC) is bad?

 

yeah, with the car not running press hard and release the brake pedal a half dozen times to vent any vacuum.

Then try holding the pedal down hard. If it continues to move (even slightly) then it is either air in the brake system or a bad master.

If it doesn't move and holds tight, then chances are the problem is the booster.