A little while ago I spent a day sorting out my self-levelling rear
suspension on the wagon, and thought I'd share a few details with the list. The car is a 1979 W123 300TD OM617.912. I gather the same principles apply to W124 and W126 models.
The problems were:
1. Spheres (MB calls them "pressure reservoirs") were shot, suspension was very hard.
2. Both before and after replacing the spheres, the rear suspension sagged within a few minutes of shutdown, or even while running after highway driving on warm days. The problem was an internal leak in the self-levelling valve, which maintains a base pressure at all times even without cargo. If your car droops it is NOT the rear springs, it's the valve failing to hold the base pressure. The steel springs are NOT meant to hold the car at driving height without some hydraulic assistance, so the car SHOULD droop if there's insufficient pressure in the hydraulic system.
As a preliminary note, machined aluminium body of the self-levelling valve was severely corroded where it came into contact with the steel bracket that is used to attach it to the car. It looked like an aspirin that had been dropped into water, presumably because some salt or other corrosive matter had nestled itself between the bracket and the valve body. The corrosion was so severe that it bent the steel bracket, deforming it by nearly a 1cm. It may be worth taking a look at that valve if you have a car with self-levelling and there's salt on your roads.
Replacing the spheres was very easy. You WILL need a proper brake/flare line fitting wrench to disconnect the hydraulic lines (11mm on the spheres); they are very tight and are easily rounded with a normal open-ended wrench. It is important to clean the entire area around the lines and fittings before beginning. I used brake cleaner, but wear eye protection and make sure there's lots of air moving or you'll get sick (ask me how I know). A lot of crud may be sitting on top of the spheres and brackets, and this will rain down into the hydraulic lines if you do not take precautions in advance. It's a good idea to use plastic bags and elastic bands or some other method to cover the line ends.
There is a (small, expensive) repair kit for the self-levelling valve
assembly, (MB part # 000-586-00-32, about $100CAD from the dealer), which includes a new valve piston, springs, ball bearings, o-rings and copper sealing rings. Give your VIN when ordering the kit, as there are different versions of the valve even within the W123 series.
The removal of the valve is very easy, but again it is critical to have
proper line fitting wrenches to remove the hydraulic lines. I believe the sizes on the valve are 11mm for lines to the spheres, and 13mm for the pressure input line. Disassembling the valve is also quite straightforward (there are only about five pieces; just be meticulous about remembering where things came from and their relative configuration. Having the MB manual drawings of the valve assembly is very useful in ensuring that everything goes together properly. Take particular note of the relationship of the regulating disc to the attached control arm, which must be detached to permit disassembly and cleaning. There are two ball-bearings which act as check valves (included in the repair kit). These will fall out upon disassembly. A third ball-bearing valve is located in the overpressure relief valve which cannot be disassembled and thus remains fixed to one of
I used varsol to clean the outside of the assembly before dismantling, then fresh varsol to clean all the pieces once disassembled. A toothbrush seems to work reasonably well as a cleaning tool. Final rinsing was done with brake cleaner, which leaves no residue but is pretty toxic, so make sure you have ventilation. I bathed components in hydraulic fluid after drying and then re-assembled. There's a note in the MB manual about not tightening the two halves of the valve until it is under pressure (there's a large o-ring that seals the two sides). I basically ignored that, since it's not possible to get a wrench into the space to tighten things after the fittings have been attached. No leaks so far, and the valve works perfectly.
Be VERY careful upon reassembly that you do NOT cross-thread the hydraulic fittings. If there's any resistance, do not force the connection. These are straight-threaded connections, and so must be perfectly aligned or you will strip the threads. Given that the fittings are steel and the valve body is aluminium, the expensive-to-replace valve assembly will lose to the fittings. I found it necessary on one line to disconnect it at the sphere end, fit it into the valve assembly, and then re-attach at the sphere end. The spheres are relatively easy to move and angle to facilitate the introduction of the fitting, whereas the valve is more cumbersome/rigid.
While you're at it you should drain, remove, and clean the reservoir and replace the reservoir filter (MB P/N 002-184-55-01 for W123 models, about $10 from the dealer). Note that the filter is in the RETURN stream of the system (shoot THAT engineer), so any crap in the reservoir can flow unrestricted into the pump and system. I'm going to install a filter on the pump-input side; anyone who has done this and can suggest a filter p/n etc: I would appreciate the info.
Once everything is hooked up again, you should be able to start the car and watch it rise to normal operating height EVEN IF the actuating lever on the valve is set to release the pressure in the system. I left the actuating lever disconnected initially so that I could verify the function of the valve etc. The car should maintain a basic height/pressure with the lever in the neutral or "release" positions. Moving the lever to the "fill" position should result in an even higher ride (unless a corresponding weight has been added to the car).