1982 300TD rear suspension

The car sags in the rear end after it's been parked for a day or so. After a couple of days it's sagging all the way down. I replaced the reservoir tank and a line that were leaking slightly but that didn't
help though the fluid doesn't leak anymore. The rear raises back up after the car runs for a couple of minutes and the ride is just fine (I replaced the accumulators about a year and a half ago and they seem fine). It stays up for a while after it's parked but slowly droops.
My guess, based on the symptoms, is that there may be an inline check valve that is defective and the system is leaking back into the reservoir. Is there such a thing and, if so, where?
What are the other possibilities?
Thanks in adance!
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Isn't there a height sensing valve?
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It is probably the valve known as the "level controller." I also have a 1982 300 TDT. I did not understand the leveling system when I bought the car 15 years ago. One of the struts was leaking, and I lost all the fluid through that strut, and I ran out of fluid in the system when on a long trip on a very bumpy, unimproved road.
This ruined pretty much every component in the system, including that valve and the pump on the engine. So I have replaced all those parts. The indication that the valve was ruined was just as you describe. The system sagged when the vehicle was parked.
Here is a link to the complete description of the system from the Mercedes bench manual.
http://inyopro.com/suspension_level_control.pdf
Those valves are not cheap. You may be able to find a used one that is good. My replacement valve has been fine for around 250,000 miles. I think it was $454 at the dealer. I have not found one aftermarket, but you may be able to get a used one from this place, which I have not tried, but claims to be able to supply 300 parts used.
http://www.automotix.net/partshotline-mercedes_benz-300e-suspensiondrive-requests.html
When my pump failed what failed was the seal that keeps the hydraulic fluid isolated from your engine oil. I noticed that when I put that very thin Mercedes hydraulic oil that is recommended for the load leveler into the reservoir, the fluid level would drop and my engine oil level would go up. It took me a while to understand what was happening. Once I understood that I started putting 15-40 Delo 400 engine oil into the load leveling system so at least I was not thinning out the engine oil. Eventually I replaced the load leveler pump. I got a used one, and it's been good for a long time. New they are $1000. The struts, and I had to replace them both, are around $350, each.
Anyway, after I got the system working again with all new parts and sealed, I switched from engine oil to automatic transmission fluid, which has anti foaming properties that engine oil does not have. I do not live in an extremely cold climate, but it is often 15 degrees overnight here in the Eastern Sierra. I have traveled to Minnesota in the winter with automatic transmission fluid in the load leveler, and often parked at Mammoth Mountain ski area all day and I have never had a problem. I have driven the car with automatic transmission fluid in the load leveler for over 200,000 miles with no ill effect.
I would recommend using ATF instead of the very expensive hydraulic oil sold at Mercedes. You should change it annually, and replace the filter when you change the fluid. The "accumulators" are spheres that have a bladder in them. On one side of the bladder there is a charge of nitrogen. On the other side is a chamber that is part of the hydraulic fluid path of the system. When you hit a bump, the fluid pressure rises, compressing the nitrogen in the gas side of the sphere. This is the only "give" in the suspension and is what softens the ride, providing the function of a normal shock absorber.
The bladders in those spheres fail eventually and you have to replace the accumulators. Those are really the only parts of the system you have to replace with any frequency as long as you keep the fluid level up and the fluid changed so you have fresh fluid with its lubricating and cleaning characteristics in place. You can tell when an accumulator has failed because the car will start to bounce more on one side when you hit a bump. If they both have failed your car will drive like a kangaroo.
A guy asked if he could take pictures of my 1982 300 TDT at the Post Office the other day. He has a 300 SD and a D and a CD, but he said of my wagon, "That is the creme de la creme." I showed him my newly remanufactured engine that I had done at Metric Motors last summer that has only 8,000 miles on it and all the brand new components of my newly completely rebuilt air conditioning system. I even replaced the evaporator! That is a 15 hour flat rate job!

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Thanks! Very helpful. Reading this I am certain that the level controller is the culprit as you suggested. The check valve must be slightly leaking back into the reservoir. The leak is slight enough that it doesn't cause a problem when the pump is running (under power).
As far as I can tell, there would be no harm in just living with it until/unless it gets worse, i.e. is noticeable when driving. Given the cost, I'd rather wait until then to replace it. Is there any danger that waiting will cause other problems?
Thanks!
PS heav, do you have any pictures of your restored wagon online? I was in process of restoring mine when I was rear-ended by a 15 year-old girl who took her Dad's giant pick up without him knowing. What a mess! Anyway, the insurance company gave me about $3000 and I bought the car back for $300. I replaced one taillight and hung a new hatch on myself which covered most of the damage (there is a dent on the rear quater panel just beside the tailgate). At some point I may get back to full restoration but aside from having a mismatched tailgate my 300 runs like a dream.
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http://inyopro.com/dusty_benz.jpg
I make my living as a scenic photographer, and this picture was taken a few years ago when I had driven out to Toroweap at the Grand Canyon and back on 120 miles of unimproved road. I have a 4Runner I use for trips like this now.
Paul

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runbiodiesel wrote:

I was thinking about this earlier today, as my car will droop a bit when it sits, but it pops right back up when I'm driving so I'm not terribly worried about it...
-tom!
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There is a problem with allowing the car to settle when you park it. This puts excessive stress on your rear coil springs and will result in them becoming weak over time. I also replaced the coil springs in my car.
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My friend with 04 CL500 told me when he is idling... the car goes down... I couldn't believe it... so it is the checkvalve right?
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Several mechanics told me it was the check valve when my car exhibited this behavior. I replaced the check valve and the car stopped sagging.
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How much did that check valve cost you and was it easy to install?
Dealer item only right?
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I actually bought mine through a friend's garage and he gave me the mechanic price, but he got it from the dealer in Seattle. That was at least 15 years ago and I think the valve was $454 or something like that.
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I don't suppose they're rebuildable? Seems like a leaking valve must have seals that could be replaced.
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I just learned another way that your rear suspension can sag when the car is parked (not running).
If there is a leak in the high pressure side of the system, the car will sag when parked. I had a loose fitting on the hydraulic line that comes out of the pump on the engine and I was losing fluid and the car was settling when parked too. I tightened the fitting and it appears to have solved the problem.
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