The
Problem:
A
leak
of
power
steering
fluid
with
no
apparent
path.
Hoses
look
satisfactory
and
nothing
apparently
wrong
with
the
steering
gear
box.
No
fluid
inside
the
belt
pulley
on
the
pump
shaft.
Applicability:
Had
this
problem
with
my
'79
300SD
as
well
as
with
the
'87
560SEC,
and
will
therefore
assume
that
most
(all?)
MB
have
a
potential
for
this
leak.
Tools
used:
17
and
19
mm
wrenches
for
the
power
steering
lines
on
the
pump
13
mm
socket
for
the
bolts
holding
the
pump
in
place
6
mm
allen
wrench
for
the
belt
tensioner
(works
really
great
with
a
battery
powered
electric
drill)
The
process:
Opened
the
top
of
the
pump
and
used
my
vacuum
pump
and
brake
bleeding
jar
to
remove
the
Power
Steering
Fluid
(PSF).
Put
the
lid
back
on
to
maintain
cleanness.
Removed
both
hoses,
plugging
the
holes
in
the
pump
and
covering
the
hose
ends.
The
hose
with
the
19
mm
fitting
was
easy
to
remove
with
the
pump
installed
on
the
mounting
plate.
The
hose
with
the
17
mm
fitting
was
easier
to
remove
once
the
pump
had
been
unbolted
from
the
mounting
plate
and
pulled
up
out
of
the
engine
bay.
A
17
mm
crowsfoot
might
have
worked
but
I
didn't
have
one.
The
o-ring
in
question
is
behind
the
cover
on
the
back
of
the
pump.
The
back
cover
has
an
internal
spring
that
presses
it
against
the
large
clip
holding
it
in
place.
A
large
(8"
or
larger)
c-clamp
can
be
used
to
press
the
back
cover
into
the
pump
enough
to
unload
the
clip
(1/16"
is
enough).
On
the
300SD,
I
placed
the
pump
on
my
floor
jack
pulley
down,
placed
a
large
socket
on
the
back
plate,
and
jacked
the
pump
up
under
the
front
bumper!
Crude,
but
it
worked
to
unload
the
cover
and
allow
the
retaining
clip
to
be
removed.
Remove
the
retaining
clip
and
the
back
cover.
I
needed
to
tap
the
cover
with
a
plastic
hammer
to
allow
the
internal
spring
to
push
the
plate
out
of
the
pump.
The
cover
is
over
3/8"
thick.
Remove
the
internal
spring
and
notice
that
the
small
end
goes
over
a
knob
inside
the
pump
and
the
larger
end
pushes
on
the
back
cover.
These
are
the
only
parts
"loose".
Nothing
else
will
fall
out
of
the
pump.
The
o-ring
is
in
a
groove
around
the
outside
of
the
hole
and
seals
on
the
rim
of
the
cover
plate.
The
access
hole
in
the
back
of
the
pump
is
2.680"
(6.80
cm)
in
diameter
and
the
o-ring
was
0.102"
(0.250
cm)
in
cross
section.
I
went
to
a
local
hydraulic
equipment
supplier
and
they
made
me
an
o-ring.
I
took
the
old
one
in
to
them,
they
found
some
0.103"
stock,
cut
the
old
o-ring
in
two
and
matched
the
length
with
their
stock.
A
dab
of
cyanoacrylate
glue
to
join
the
ends
and
we
had
an
o-ring.
I
had
them
make
one
a
little
larger
diameter
and
a
little
smaller
"just
in
case".
I
would
have
preferred
to
use
Viton
for
the
o-ring
but
it
was
unavailable
so
we
used
Buna-N.
None
of
us
knew
the
chemical
composition
of
PSF
and
couldn't
verify
that
neoprene
would
be
satisfactory.
If
it
leaks
again
in
several
years,
no
big
deal
-
it's
fairly
easy
to
replace.
The
300SD
o-ring
was
about
1
1/2"
in
diameter
and
sealed
against
the
flat
portion
of
the
back
cover.
They
may
be
different
for
different
Mercedes,
but
the
repair
process
was
the
same.
Mercedes
does
not
sell
the
o-ring
for
this
repair
but
call
around
to
find
your
local
hydraulic
equipment
repair
shop
and
they'll
have
or
make
your
new
o-ring.
Slathered
the
new
o-ring
with
PSF
and
put
the
pump
back
together.
When
tensioning
the
pump,
remember
that
the
allen
head
cap
screw
for
the
belt
tensioning
mechanism
is
threaded
into
a
breakable
plastic
block
that
pulls
up
the
pump.
It
took
less
than
one
quart
of
PSF
for
this
task.
Filled
the
pump
and
started
the
engine.
Ran
it
for
about
ten
seconds,
move
the
steering
wheel
just
a
little,
then
stopped
the
engine
and
refilled
the
reservoir
(down
by
half).
The
manual
says
that
the
system
can
be
bled
by
lifting
the
front
wheels
off
the
ground
and,
with
the
engine
running,
cycle
the
steering
from
stop
to
stop
several
times.
I
did
it
with
the
wheels
on
the
ground
and
all
was
well.
I
now
have
a
very
clean
pump
and
no
more
leaks.