Okay, you probably have to buy a new brake booster (expensive) like Walrus writes (I'll leave it to him), but sometimes it's a little thing. Why not try a "way out there" but very cheap possibility before you spend the big bucks? Sure, you haven't mentioned (haven't experienced?) a hard pedal, but let's try this anyway -- can't hurt.

The booster has a one way valve between it and the vacuum feed to it -- it's a little right-angle fitting (usually white plastic) that plugs into the booster on it's face on the right (passenger) side. Pick up a new one at your dealer (don't use a p-n-p, it may be worn, too), only a few dollars. Maybe it will help -- what have you lost?

If it doesn't work, then mortgage your house and buy a new brake booster, probably.