I don't believe in telling people what they need to do, I just share my experience with the same situation.
My trunk was not going down with just one push of the button on the trunklid, I took out the paneling on the passenger side of the trunk and behind there, I found the reservoir that holds the hydraulic fluid for this to work correctly. I proceeded to take off the two nuts that hold it in place, once I was able to do that, I turned the reservoir upside down so it could be filled. You'll see a lot come back into the plastic cap once you turn it upside down but that's fine, there's still room to add plenty more.
Take a small screwdriver and take off the clamp that holds the clear sump so you can proceed to refill it. Once off, fill it with the correct fluid then place the hydrolic motor back on top. At this point, you should have not spilled one drop and it should be filled to the "max" line on the clear piece. Make sure the band is tight before flipping it back upside down again in order to mount it.
Once you finger tighten the 2 nuts/bolts that hold it in place, close the trunk fairly quickly without locking it and then lift it back up manually and do that 2 to 3 times. This is the way in which I bled the system and thoroughly flushes the fluid all the way through the hydraulic pump. This is SUPER IMPORTANT before you have the pump try to close the trunk on it's own.
Once that's done, you should be able to open the trunk all the way, push the button and it should work perfectly. If not, continue to open and close the trunk so it bleeds the setup and gets all the air out.
Hope this helps... I actually found someone on this site who did a write up on this and I couldn't find it so I figured i'd type it out again with what I did. Good luck with the fix.
I tried the SMG Hydraulic Unit Fluid - CHF 11S BMW E60 M5 S85 5.0L but then, the BMW Genuine Convertible Pump Hydraulic Fluid (54 34 0 394 395). The 54 34 0 394 395 was the best to date, it ran very thin and once I bled the line... never an issue again. Hope that helps.
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