Craig, your master cylinder is exhibiting "bypass."  The piston is worn or the cylinder itself is worn, and allows fluid to bypass the piston.  The pump-up phenomenon is characteristic, as the parts expand a bit with frictional heating, or so I have been told.  At any rate, the symptom is classic.

The part should be warranted by the seller.  A bad new part is not unheard of.

mcneely

---- Craig Cowan <phishman068@GMAIL.COM> wrote:
> Hey gang.
> Last summer I had a problem with my master cylinder in my syncro, or so I
> thought. The pedal would "Sometimes" go all the way to the floor but could
> pump up to build pressure. Other times it would work fine.
> Anyway, I replaced the master cylinder, bled the system and all was well.
>
> Well now about 1000 miles later, i've got the same symptoms occurring. The
> fluid level has not gone down, so I don't have a leak. It feels as though
> something has blown out and is allowing a piston somewhere in the system to
> move freely. I'm thinking it's the master again, but it sure is weird to
> kill two as quickly as I have.
>
> Any thoughts?
> When the pedal works, it's firm and functional. But sometimes I get a firm
> pedal, and while keeping my foot on the pedal it slowly moves towards the
> floor.
>
> -Craig Cowan

--
David McNeely