what is exactly this bomb?

[Decsi Miklos <MDecsi@pgsm.hu>]

Hello.

As some of you know  the rear brakes on my '83 80 Quattro don't work. When I
inspected the brake system I found out that the problem can be either at the
master cylinder or at a misterious device just below it. I think it is what
you call the bomb. I talked to a mechanic and he told me that this is some
kind of a brake pressure regulator.

 Question: Is this thing really called as the bomb?

 If yes, then how does it work? 

 If no, then what the hell is this? 

 It is not connected mechanically to anything. Only pipes in and pipes out.
I have never seen a regulator like this. Another strange thing is that
pressure regulators are normally at the rear wheels, but it is just under
the master cylinder... 

 I don't understand, can someone help me?

Decsi, Miklos