The power steering reservoir has sort of a filter inside, the return line comes in the middle, feeds the ATF inside sort of a porous ring which transfers the fluid to the container to be fed towards the pump by the outer line.

The outer connection is ID 15mm, and feeds oil from the reservoir to the pump.

The center connection is ID 12mm, and is the return from the steering via the cooling pipes in the front of the car.

The power steering reservoir is used in many other cars:

Ausgleichsbehälter für Servolenkung
Power Steering Reservoir

Passend für Porsche 944,928,924
Fits Porsche 944,928,924

Vergleichsnummer : 32 41 1 097 164
part number : 32 41 1 097 164

Hersteller : APA
Producer : APA

BMW
318i 1984-1998
318is 1991-1997
318ti 1995-1999
323Ci 2000-
323i 1998-2000
323is 1998-1999
325 1986-1988
325Ci 2001-2005
325e 1984-1987
325es 1987-
325i 1987-2005
325iX 1988-1991
325is 1987-1995
325xi 2001-2005
328Ci 2000-
328i 1996-2000
328is 1996-1999
330Ci 2001-2005
330i 2001-2005
330xi 2001-2005
524td 1985-
525i 1989-2002
525iT 1993-
528e 1982-1988
528i 1997-2003
535i 1989-1993
540i 1994-2003
633CSi 1982
740i 1995-2001
740iL 1995-2001
M3 1988-1999
X5 2000-2004
Z3 1996-2002
Z8 2000-2002