Hydraulic clutch question

[steveb@falcon.kla.com (Steven Buchholz)]

While driving the '88 5kQW the other night I experienced the "clutch pedal
goes to the floor and won't come back up" problem.  Oh, boy ... I get to 
practice my driving without a clutch technique!  It appeared that the prob-
lem was that air got into the lines due to the reservoir being low, so 
last night I topped up the brake fluid and started the process.  Nothing 
came out of the slave cylinder and I heard a strange sound in the brake 
fluid reservoir when the pedal was raised.  I took off the fluid supply 
line to the clutch to try to figure out what the noise was ... when the 
pedal is raised the master cylinder pumps fluid back into the reservoir!  
... at a reasonably high pressure I might add.  It seems as though a seal 
or valve in the master cylinder is screwed up.  I was wondering if this 
could possibly be an effect due to air in the lines or if I have to bite 
the bullet and rebuild/replace the MC.

TIA!
Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)