Audi A4 2001 Owner's Manual page 31

Two-stage ABS return pump
Working chamber 1
The piston of the two-stage ABS return pump
is stepped and has a double-acting function
within two working chambers.
Suction is effected in two stages; brake fluid
being drawn through the suction line during
each piston stroke.
Function:
If the piston moves from BDC to TDC, the
brake fluid in working chamber 1 is com-
pressed whereas working chamber 2 is simul-
taneously under suction.
If the piston then moves from
TDC to BDC, the brake fluid which has been
drawn into working chamber 2 is forced back
into the suction line against the inlet valve.
Working chamber 2
As the entire suction volume is supplied
almost continuously, the maximum suction
flow rate is significantly lower, which dimin-
ishes the suction pressure and prevents
cavitation.
Thus, a quick pressure build-up is ensured,
even at extremely low temperatures.
In working chamber 1, brake fluid is now
drawn via the open inlet valve, out of the suc-
tion line and out of the connection line to
working chamber 2.
The suction flow rate in the suction line is
diminished by that amount which flows back
out of the connection line from working
chamber 2
(drawn in during previous working operation).
Suction volumes
UT OT
UT OT
UT OT
UT OT
SSP254_096
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