Start by checking all the wheel cylinders for leaks. It sounds like you've lost pressure to half the system for some reason, or one chamber of the master cylinder has failed. A bad booster would result in a hissing noise, a very hard pedal, and possibly a rough-running engine if the vacuum leak was bad enough.

The pedal going over halfway to the floor is a dead giveaway that one end of the brake system has lost pressure. That's how it's supposed to work.
Jerry

Last edited by Hotrod Lincoln; 09/09/2008 5:36 PM.
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