Changing out all of the little filters in the hydraulic fluid lines.
The parts CD shows four separate filters for AHK equipped 850's.
The first one is the one we all know about, located at the bottom of the fluid cannister in the engine compartment.
This one is the primary filter and should be serviced at some point. You need a turkey baster to suction out all of the Pentosin in the cannister. Then remove the screen and other attaching parts, lift out the dirty filter, and replace with the new one. Reinstall hardware in reverse order of removal. Do not refill yet.
The second filter is located underneath the front engine screening tray which is attached to our cars between the radiator and engine block (i.e., the engine accessory belt cover). There is a small filter located in this housing.
Here's a new one compared to the filthy one removed:
To access the filter, first remove the bolt attaching the support bracket seen at the top right. Then using the appropriate size socket, unscrew the small cannister from the bottom. A small amount of fluid will be trapped in the cannister, so have a receptacle in hand to catch it unless you like cleaning hydraulic fluid off the floor. Note that every 850 should have this front small filter, and that BMW service training materials indicate that it is important to periodically R&R this one.
For those 850's equipped with AHK, the rear pump unit contains two filters. The main filter and it's housing is shown here.
A comparison of a clean filter versus the removed filter:
The housing screws into this receptacle on the main AHK pump unit:
The final filter is located on the upper pump assembly of the AHK beneath the small rectangular cover:
With the cover removed, it looks like this:
This was a little tricky to remove since it is installed upside down in the housing. I used a small paper clip with a hook I bent into it to catch the lower edge of the metal filter bottom to pull it out. BTW, this is the exact same filter as was used at the front of the car. A separate service gasket set is available to seal this one back up.
After replacing all of the filters and closing everything back up, it's time to add replacement Pentosin. Remember there will be a fluid deficit in the small filter holding cannisters, so you will need to fill it up, start the car up to let the pump circulate the fluid, and then shut it down and recheck the level in the cannister. I had to top mine up to the required 20mm below the rim level, which is just slightly above the screen inside the cannister. Here's a picture of the fluid after replacement, topped up to the correct level:
The fluid removed was quite black. I'm not really sure if it's black because of dirt in the system, the hoses losing some of their linings, or the mechanical action of the pump. But whatever the reason, I'm thinking a 6 year old car was a perfect candidate for some clean filters and fresh Pentosin.