Welcome to the Group!
I assumed you checked all the lines and looked for leaks. I just bled a system that had been sitting for a long time and when I started pumping fluid through, I found a leak in one of the rear flex lines. Once I got everything bled, I found that one of the rear wheel cylinders was leaking, so I was losing fluid that way. I'm sure the more expert members will chime in with better advice, but I just did this two days ago, so it was fresh on my mind.
Dan
1965 230 SL Euro 4 speed
2004 230 SLK