Take a very close look under the dash.  They can leak out of the back of the master cylinder, and as with the caliper example, leave only small traces.   These traces can run under the matting and carpet, leaving very little traces.

   As for not having rear brakes before I found the current problem,I wasn't having any truoble stopping. Are you all saying that  putting the car on jack stands can cause other problems? It's been a little to cold  for me to work on the car(down side of not having a garage) Also I did check under the dash  where the brake pedel comes though the fire wall.

      No Rick, jack stands are a great tool to use and are a great safety helper instead of working underneath a vehicle using only the jack. You never want to trust the jack only. A good Corvette shop manual should show you where you should place the jack stands. If you don't already have one try a local auto parts store. Or check out your local library.
      You should hear me piss n moan about my 3/4 size garage to work in. I can't imagine not having a garage at all!!!   And trying to do it during the winter!!!!!!!!!!!!! You're a better man than I am. Good luck again.

P.S. Looking at the picture of your car in the driveway, it appears as though it's on a hill. I hope whenever you jack it up it's on a flat level surface?

Larry is right, if the 75 has the porportional valve, it should have set to block the rear and then turn on the Brake lt

 It did turn the brake lite on.Howevevr I drove the car the weekend before I found the problem,and I put the car in the driveway.When I did this I had no probelms stopping the car. As my picture shows  my driveway is on a angle,and right now I have  my TA in the driveway. I backed in put brakes on and parked the car without hitting the TA. the next weekend no brakes and lite was on.

I beleive if you replace the fluid and do a brake bleed you will discover the problem. if you bleed and things reset and you see no evidence of fluid at the calipers and lines, you would be ok. if the fluid loss occurrs again it would have to be in a hidden spot like the booster or were the metal lines are not exposed

  That my plans ,but I have to waiy for the weather to get a little warmer.I looking at repaceing most of the brake system,because I don't know how long since it's been done or how old the parts are. Just a quick question:the front brakes do they run off the front bowl or the rear bowl,I have been told two different ansewrs.The manual I have dosn't say.tinbender2009-02-23 07:38:58

You should be able to see where the front lines come into the proportioning valve and from there directly into the master cylinder. Typically, the larger sized bowl is for the front brakes.