I'm no super tech but I'm betting that the rear portion of your brake reservoir is empty!

MB of this vintage have a reservoir with two sections, the rear section is filled by overflowing the divider between the two sections.

Get a good flashlight and you can more easily see the fluid level in that rear section of the reservoir. Sometimes wiggling the plastic reservoir left and right helps the rear section fill a bit faster.

Give the reservoir a good examination first before you start jumping to conclusions that "might be" despite any lack of direct supporting evidence.