Hydraulic system check valve?

[Michael Spiers <michaels@greeley.sykes.com>]

I started the replacement of the bomb in the '84 5kS this weekend.  In
the middle, I went to a friends house to discuss life.  This friend also
happens to be an authorized Audi mechanic, so (after he gave me the
booklet I've been searching all over the place for) we started talking
Audi's (what else do you talk to an Audi mechanic about?) and I told him
what I was doing to Ann's 5k.  He told me to check what I *think* was
called the check valve.  We went out to my TQ in his drive, and he
pointed out what he was talking about.  He pointed to the return hose
from the block behind the master cylinder that returns to the top of the
hydraulic fluid reservoir, and said "the way to check it is to take that
fitting off, and start the car.  If no fluid leaks out (don't _touch_
the brakes!) the valve is OK, if some leaks it's bad."  What valve was
he talking about?  It _is_ the block of aluminum between the master
cylinder & the firewall right?  What's that thing called?  If this thing
is bad, is it going to spray Pentosin from to Germany?  TIA-

-Mike
MKSpiers@frii.com
MichaelS@greeley.sykes.com
'87 5kCS TQ (Fort Collins, Colorado)
'84 5kS (Boulder, Colorado)
'90 80 (Bethesda, Maryland)