Clutch slipping some; master cylinder / relief valve?

[LugNut]

The 2001 SE Sedan, with the SPI that I rebuilt and put in a new clutch about a year ago, now has a slipping clutch. The driver (not me) is very experienced with manual transmissions, so I thinking it's not from driver error.

I just had the car for a bit this morning. There's no leakage of oil from the vent at the bottom of the bellhousing, so a bad engine rear seal is not likely.

From the FSM, the hydraulic clutch is not adjustable. Clutch pedal operates correctly. Hydraulic fluid in the shared reservoir (brakes and clutch; not a better idea!) is clean and full.

From a test drive, it is slipping some in all gears, but does slowly engage. If not a bad clutch set ($700+ by a shop), my other guess is a bad/clogged hydraulic pressure relief that is slowing the release of the throw-out bearing. I'm also assuming the relief valve is part of the master cylinder.

Sound possible? Or better yet, solved the same yourself?

A new master cylinder is only $35, but a pain to install. I see this good thread on how to do it:

http://www.focusfanatics.com/forum/showthread.php?t=243148

Thanks!

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