Thanks for the replies.
I never noticed there were two separate reservoirs until yesterday when I got a chance to take off the wheels and take a better look. The one in the front was full but the one in the rear was empty so I was mistaken into thinking they were both full.

I checked the rear lines and the left side flexible line had some minor cracks in it but no sign of leakage. I plan to replace them all anyway as I dont know when they were last done.
Would an empty rear reservoir correspond to a rear brake line leak?

Would this possibly rule out the mater Cylinder as the culprit? (the Mercedes parts dealership quoted me over $500 for a new MC!) I found brand new ones online and from Canadian sources for as low as $60. Not sure if thats a gamble though. Is there an obvious quality difference? Sure is a price difference.

I filled up the reservoir again and bled the line I took off and it works fine again, however I know this is temporary. I'm just waiting for the parts to be shipped.

I alos was considering getting new brake pads as they may need replacing withing the next year. But when I was about to choose them I got 3 different versions. One that says for ATE Calliper 123, one for a Bendix 123 and the other simply said it was for 123 models.
All 3 options had an OEM selection. I wasnt sure if thre was a big difference between the models if they are all for 123s. The callipers appear to be original so I was just going to go for the general 123 option but perhaps someone could shed some light on the differences?