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DIY refill Mercedes Benz R170 hydraulic roof fluid
Thanks to user Thud57 for this great information.
Filling the roof pump is not as complex as it appears. Don't waste your time (as I did) with the fill screw on the side of the pump facing you. Here is what you need to do.
1) depressurize the system using the hex wrench that is attached to the plastic access panel. If it isn't there you'll have to find one that fits - I'm sorry, I forgot the size. Follow the owner's manual instructions and rotate the brass colored screw (I don't have it in front of me, but I think it says counterclockwise) all the way.
2) on the top of the pump where all the hydraulic lines go in there is a filler hole that makes life so much easier. You can't really see it very well because it lies on the upper far right hand side on the top of the pump in that same area. I think it takes a 5mm hex wrench. If you feel with your fingers you will find the cap screw there on the far side of the lines. It loosens counterclockwise and it may take some effort as it is torqued down kind of tight. BE VERY CAREFUL WHILE LOOSENING. Once it is loose remove the wrench and unscrew it with your (dry non-greasy) fingers making sure that when it is finally free you don't drop it in the pump access cavity. If you do you will never see it again as it will drop way down into the car body interior.
3) Home Depot sells 1/4 inch OD rubber tubing (20ft for 2.00) get it and find a baby syringe at a grocery/drug store. The nipple of the syringe should fit perfectly into the tubing.
4) cut about a 2 ft length of tubing, insert it into the filler hole (you should be able to go in a few inches), take out the syringe plunger, pour a portion of the special hydraulic fluid into another small shot-glass size container and pour that into the syringe that is attached to the hose a little at a time. Repeat procedure until the fluid level is between the two lines you see on the plastic pump reservoir. It shouldn't take much fluid.
5) once at the level VERY CAREFULLY reinsert the filler screw and hand tighten with the hex wrench. Now you're good to go.
6) find out why you are loosing fluid - I understand headliner droop is a tell-tale sign. Also look for run lines inside the trunk under the two roof actuators.