300CD79.

The self levelling system consists of a hydraulic fluid reservoir and an engine driven pump, hoses and hard tubing as required to route the hydraulic fluid, a valve and mechanical lever to sense when the suspension is compressed or extended, and position the valve to either pump the system up or lower it in response, a pair of spheres pressurized with Nitrogen on one side of a membrane, the other side is connected to the hydraulic fluid system, and some hydraulic cylinders (one at each rear wheel).

When the engine is running there is a source of pressurized hydraulic oil at the control valve (mounted between the rear suspension and the car's body). If the car is loaded down the valve is openned so flow can go from the pressurized source to the hydraulic cylinders and raise the car. If it is high the valve bleeds fluid off and returns it to the reservoir. If the car is just right the valve sends the fluid back to the reservoir.

The Nitrogen pressurized spheres serve as the receptacle for fluid when the suspension compresses due to dynamic driving loads (the pump and control valve are not fast enough nor are they intended to provide responses in the times needed for wheel position/response to road irregularities and chassis responses to driving inputs) and a source on the return stroke of the cylinder. If this component fails the sphere fills with fluid and then there is no place for the fluid to go when the rear wheel wants to move relative to the body. The car bounces and drives very poorly over bumps.

Since the system is "solid" except for the spheres, adding fluid to the system volume forces the hydraulic cylinders to extend to make room. This lifts the car.

The most common things to fail are hydraulic lines, the hydraulic cylinders at the wheels (look a little like shock absorbers) and the spheres. I have only recently heard of the pumps failing. The fluid must be MB's product or all bets are off. The fluid is often neglected and left to turn dark brown. It is a light gold color when new and should be changed regularly, along with the filter in the reservoir.

So, look for signs of leaking, push down on the rear end of the car on each side and look for normal shock absorber type responses, and look to see the car is level with no rear end load and with 3 or 4 people sitting on the rear bumper with their feet in the air.

I do not own a 300TD so I cannot answer many more questions. I think rust is the biggest issue for these cars and you have that covered. The suspension system is the next biggest issue as I see a lot of them driving around dragging their rear ends, and the system is often a topic on this forum.

So, good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)