have a 1980.  after starting car from storage, brakes are really low.  have to push the pedal(it's real spongy and soft) all the way to floor before they grab.

Just because there are no visible leaks doesn't mean there isn't one.  One flaw in the brakes of C3's is not design, but that they were cast iron and could corrode quite readily if not used regularly.  While a C3 is in storage you should pump the brakes at least every couple of weeks to keep the seals lubricated. 

On the '78 I had I had a problem with low pedal and air in the lines.  No amount of bleeding the lines corrected it for more than a short time.  It turned out a caliper seal was leaking and instead of leaking fluid, it was sucking in air, creating the problem.  Once found, it was an easy fix.

You brake system may be in need of a total rehab, depending on how long since it's been done previously.  Have the entire system checked out...hoses, calipers, pads, etc., for safety sake.  That doesn't mean everything has to be replaced, just thoroughly checked.

I had no sign of leak until I put it on stands with wheels off and low and behold the front caliper was leaking...and another caliper was sucking air as mentioned above. I replaced alot that probably didnt need replacing, but better safe than sorry. I basically did a complete rework of the system. If you have any questions I may be able to help...?

At the very least you have air in the system. Try bleeding them first, and if that doesn't bring the pedal up, even if it goes back down overnight, then replace the master.
If they do come up, but it goes back down after driving, then you'll need to rebuild/replace the calipers, and make sure the rotors are running true, and they don't have any run-out to them. Run-out will cause the caliper pistons to become air pumps, sucking air into the system while you drive.