Pressure Accumulator notes

[Bob Kunz <bkunz@hpbs114.boi.hp.com>]

There has been a lot of discussion about pressure accumulators recently. Since
I just replaced mine, I'll tell you what I found out.

First and most important, do not confuse hydraulic oil and ATF. They are
not the same thing and ATF in the hydraulic system will ruin the seals.
This WILL lead to leaks in the steering and possibly cause the check
valves in the accummulator to stick. This leads to power brake loss unless
you have a particularly good pump working for you.

The correct fluid (confirmed at both the dealer and the local German
auto parts store) is Pentosin hydraulic fluid. This is specified by
both BMW and Audi (proabably others). There are two types. Both are
mineral based although one is called "synthetic." The primary difference is
the upper bound on temperature range. For the "7.1s" fluid this is 100
c. for the "11s" fluid it is 140 c. 1984 and older model years use the
7.1s fluid. 1985 and newer use the 11s fluid. This costs about $18/litre.

If you have used ATF or hydraulic jack oil in the past, you must get this
out of the system. You can drain the reservior by disconnecting the return
line from the pressure accummulator. Gravity does the job. I used only
one litre of fluid to flush and fill mine. Don't forget this fluid also
circulates through the power steering. To move fluid here you need to 
turn the wheels lock to lock with both front wheels off the ground.

The pressure accummulator (PA) has three hydraulic lines. Two are high
(> 2000 psi) pressure lines, the third is a return line to the reservoir.
The PA is pre-charged with nitrogen. To change it...

1. Depress the brake pedal repeatedly until you lose assist. Could be 35x
   on a new PA.

2. The PA is (at least on mine) on the driver's side. Find it by following
   the return line at the bottom of the reservoir. Disconnect all three 
   lines from the PA. Fluid will come out of the return line.

3. The tip of the PA has a threaded stud. Remove the nut. Lift from the
   back at the return line connection and remove.

4. Install the new one with new compression (aluminum) washers if possible.
   These washers deform to create the seal. At least turn them around.

5. Fill (and/or flush) see above the reservoir.

6. Start the car and fill the reservior to minimum as needed. The PA will
   now accummulate fluid and it will take about a pint of additional fluid.

7. With the front of the car off the ground, turn the steering lock to
   lock to bleed the circuit through there.

8. Check for leaks.

The hydraulic pump is really two pumps in one. One runs a circuit for the
power steering the other runs the circuit for the brakes. There is a way
to test a PA by examining the pressure at which the power assist goes
away. Usually on a broken one, this is easy to do since with the engine
off there will be less than one stroke of the brake pedal before assist
is lost. On a weak PA (i.e. one that has lost some nitrogen), it could
be several strokes before loss of assist occurs. The normal minimum
number of strokes is 20. A new one could be 35. That's what mine is now.
You can decide when it becomes important to replace by measuring the
number of strokes.

Do not fill the fluid reservoir to the MAX line. The reservior will not
hold all the fluid that the PA returns to it when you de-pressurize it.

What else do you need to know?


Bob Kunz