Great pic thanks Michel.
I don't get something, where is the pickup? And how is the filter supposed to filter the oil if its up in the air high above the high-level marked on the dipstick?
By my mind, any crap in the system has to do a tour of the hydraulic system and come back to the control valve, then filtered then returned to the reservoir??? Sounds a bit dumb to me... the engine oil system makes much more sense, filter the oil before pushing it around the engine...
Just my 2c worth.
I.
The pickup is not shown in that diagram but is an angled tube that comes out of the base at about a 45 degree angle and is extended inside the reservoir so that there is a small amount - about 500ml which does not circulate but allows solids to settle.
This tube is connected to the end of a length of metal tube which acts as a cooler via a rubber hose (I replaced with transmission hose of the correct diameter)and the other end of that cooler is connected to the input side of the pump by another length of rubber hose. These lengths of hose should be replaced and new non-sharp edged clips used so that they can be forgotton about for another 30 years.
The paper filter (3h in the diagram) is in the return line from the suspension and is supposed to catch whatever has returned from the suspension before it gets into the reservoir. (Serves to protect the pump which is more fragile than the usual engine oil pressure pump)
The sump arrangement allows what gets past this paper filter to settle undisturbed in the bottom of the reservoir which is where the surprising amount of black crap is usually found.
A possible major cause of pressure loss and sluggish functioning is the other small compressed brass filter which should be present in the valve body under the connection where the high pressure line connects to the valve body on the reservoir. This is the one that was missing on my car but others have reported to have been clogged.
I would clean this in thinners or such-like and then blow with compresssed air until dry before replacing.
Bill