I recently got a new-to-me 83 300TD and have been going through it bit by bit.
One of the first things I wanted to get working right was the SLS.
I ended up digging through the archives related to the SLS and wagon suspension but I wasn’t able to find anything that directly addressed my symptoms. I found tons of great info on the accumulators, hydraulic shocks, and the proportioning valve but not much on the pump itself. So I wanted to add to the body of knowledge here in case someone encounters a similar problem.

The rear end wasn’t sagging at all, but I wanted to verify the operation of the system and make any necessary adjustments to the ride height.
The first thing I did was to check the obvious, that there was fluid in the reservoir. Then I drove it up onto ramps and got underneath and unbolted the linkage that connects the proportioning valve to the rear sway bar. I manually operated the arm of the valve up and down but there was no change in the ride height.
(BTW in my ignorance I initially almost unbolted the arm that clamps onto the SLS valve itself. I later came across a warning in the FSM not to do this, as it is difficult to realign.)

The next thing I looked at after combing through the archives, was that I was getting flow back through the return line.
With the car off I disconnected the hard return line from the top of the reservoir bottle and maneuvered a plastic container under the hard line (I cut a hole towards the top of a clean dry milk jug to allow the hard line through). I then started up the car and observed I was getting flow back through the return line - what I had gathered from my research is that the pump itself is not working if there is either no flow through the return line or if there is only a trickle. But I ended up finding this wasn’t really so.

I initially suspected a faulty proportioning valve and ordered a rebuild kit from a forum member.
I also bled the system down by putting a hose on the bleeder port of the proportioning valve and when I did that the rear end completely sagged down. I was curious and closed the bleeder valve and started the engine to see if the system pumped up again but it stayed sagged out. I questioned my diagnosis of the valve at that point and decided I should take a closer look at the pump.

I looked everywhere couldn’t find anything other than a complicated procedure in the FSM to measure the pressure being generated by the pump, but I had my friend’s wagon nearby with a functioning SLS and decided I would first just try the quick and easy thing of measuring flow through the return line on both wagons and see how it compared. I took my milk bottle and ran the return line into it from my friend’s wagon and started my stopwatch when the engine started and got out to observe the return flow. At the 60 second mark I shut off the engine with the shutoff lever under the hood. I took the milk bottle out and put it on a flat surface and marked the level. I poured the fluid back into the reservoir and then followed the same procedure on my wagon. I discovered that for the same 60 second timeframe I only got maybe half the volume of fluid returned. It wasn’t anything like a trickle, so initially I thought the pump was fine.
I measured that from the working pump I was getting 24 oz. - 3 cups - returned to the reservoir in 60 seconds. With my pump I was getting maybe half of that. Not as exact as the FSM but neither did I need an elaborate setup.

Anyway I was able to get a pump from a used mercedes parts guy local to me, and it was a very easy job to swap the pump. (As a note, before I found that that I found that merc***source.com has what seems to be a complete pump rebuild kit.)
I used a 5mm hex head socket and I didn’t have to remove the fan or the top radiator hose as some directions had mentioned.
And now my SLS works great!

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83 300TD

Thank you!
This is extremely helpful. Once all the SNOW melts away here, I plan on doing some diagnosing on mine.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."