I drive an 87 gt (automatic) on a daily basis. And the other day on my way home the brakes just gave out. My pedal went all the way to the floor and it barely slowed down. Once I got home I checked the Master Cylinder Reservoir and both sections are full. I checked under the car and I don't see any leaking fluid and no fluid was around the MC or the brake booster or the vacuum hose up front. So I don't think its a fluid leak. I'm wondering if I have an air leak somewhere or what. But if I had an air leak I feel like the car would idle different. But when I start the car it idles fine. Just the pedal goes all the way to the floor. Does anyone have any tips or Suggestions?

Okay, go out to your car and try this: with the car off, pump the pedal. Does the pedal get hard to the point that it is very difficult to press?

Ya when the car is off the pedal is hard, once the car is on the pedal goes sofft

Sudden Soft pedal AND no external leak = MC is bad or maybe you've boil the fluid.

MC then replace MC

If you Boiled fluid in street cars then fix what caliper is dragging and flush old fluid.
Get a cheap IR meter to quickly find a dragging caliper.

See my Cave, IR thermometer and Brake Fluid

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

You might have a leak. Try pressing on the pedal with the engine running, but don't press it to the floor. When this happened to me I could hear it spraying. Anyway if it's leaking you will find fluid, but probably not find it missing. The hard lines rust out eventually. That's what happened to me and that would be my guess.