Strange Braking problem

[PhoebusApollo]

First off, I searched and didn't find an issue identical to this.

I just replaced my rear brakes (OEM calipurs, rotors, pads, and goodridge steel braided lines).

We bled the brakes several times, until there was no air in the system, to the point where the fluid coming out looks "EXACTLY" like the fluid being poured into the reservoir. We even went back and did the fronts even though I didn't touch them, to make sure no air traveled down those lines.

While bleeding the brakes I would pump to press out the air bubbles and then my friend would close the bleedervalve and the peddle would be ROCK solid.

Here's where the strangeness comes in. After the brakes are rock solid, if you let them sit for 30 seconds or so the next peddle press ALWAYS travels to the floor. If you press again, it's ROCK solid, and every subsequent press is solid. But if you let the peddle return to the top of it's travel the next press will go to the floor.

After discovering this we bled the entire system again (4 pints of brake fluid in all were used). I can say without any doubt there is NO AIR in the system.

I'm pretty sure this is an issue with the master cylinder (I can't believe that air would only cause an issue with the first press and not every press), and I thought I read on here somewhere about an issue with the valve inside the MC that can be corrected. If no one is aware of this, what is my best option. And consider that the front set of these:
http://www.courtesyparts.com/Merchan...x_nms_gtrbrake

are being installed either this coming saturday, or the following saturday.

If it is the MC, should I replace with the same MC or is it better to use something from the Z32? If so what pitfalls (different lines, etc) do I need to consider?

I know this is long, so thanks for reading.