I did a quick search...didn't see exactly what I was looking for...so here'* my story:

on the '93 SSEi...brakes worked fine on the 6 hour drive home from MA/CT. I get in to test drive...pedal goes 3/4wayto the floor before getting any pressure. Checked the brake fluid - full. No signs of leakage. Son couldn't get the bleeders open, and he'* tied up with finals/work ...so we took it to the local garage for their diagnosis.

They are totally perplexed about it. Never seen this happen before....they were able to bleed the brakes - but air is definitely getting into the line somehow, yet the fluid level is correct. How can air get into a line if the system is closed?

Could it be the master cyl not operating properly? Perhaps its sucking air in that way? Besides the MC...what other things should we consider replacing or testing?

The vacuum booster is one area to consider. Anyone know the symptoms of a booster?

How about one of the brake hoses about to burst and maybe each time you press the pedal it expands like a balloon then goes back to normal when you let off?

ya the rubber brake lines could be getting old and expanding

i would think that maybe one of the seals is bad in the master cylinder, allowing some fliud to blow by the seals... other than that, the rubber hose collapsing could be the problem aswell, but the mechanic should have seen this while bleeding the brakes

I would start with the two rubber brake lines on the front calipers for starters, simply because those are items (if they're original) that have a limited lifetime anyway, and are relatively inexpensive.

Are you getting air on BOTH sides? If so, you can generally rule out the calipers and start looking at the MC.

Make sure and look around the area where the Master Cylinder meets the Brake Booster. Specifically directly underneath the MC. If you see a trail of shiny brake fluid seeping down the booster you've got a good sign of internal MC seal leakage. I've had several of these come through my shop, they're relatively simple to do. But for sure if you can pull your front wheels, or just turn them, and inspect the rubber hoses for ANY signs of wear/cracking. No big deal to change those either, but you may need heat. I usually do. Doesn't suprise me that you couldn't get the bleeders open either they have to be heated a lot also.

I believe the mechanic said "...getting air on one side...." I've got a message out to find out for sure. Will check the hoses and the MC as noted....thanks guys!

Update:

It was indeed the Master Cylinder. The driver front and passenger rear were both getting air...evidentially these are operated by the same part within the MC. Put a new one in and she'* ready to run.

As much as I believe in using GM parts, I really hate paying for them more than I need to.
For those of us tired of paying for the failing parts, here'* the number'* from Napa with lifetimre warranties.

Tru Stop TS 102714 Reman, Limited Lifetime warranty
United UP 390267 New, Limited Lifetime warranty

You do have to swap the reservoir, so it'* not just a quick bolt on. I haven't done it yet, but both of my Bonnies have weak MC'*. The 93 is a GM replacement I did 6 years ago.