What is this part?

[Lokkju]

Can anyone identify this part for me - it is the one with the two
water hoses connected to it, and the vacuum tube(?) coming out the
top...  This is a 1987 5000S Quattro - as I mentioned in an earlier
post, this just plain seperated on me, dumping all my coolent - what I
really need to figure out is if replacing the part will fix the
cooling system, or if the part failing was just a symptom of a larger
problem...

Thank
Nick

http://xs.to/xs.php?h=xs27&d=05184&f=AudiPart01.jpg
http://xs.to/xs.php?h=xs27&d=05184&f=AudiPart02.jpg
http://xs.to/xs.php?h=xs27&d=05184&f=AudiPart03.jpg
http://xs.to/xs.php?h=xs27&d=05184&f=AudiPart04.jpg