I believe the m/c reservoir on 124s is configured like the 201s, and it is very easy to run the rear reservoir dry. You can only see the rear reservoir level at the extreme rear of the res. by viewing from the "inside" (engine not fender side), and it's much tougher to read with clean fluid.

Internal baffles in the res. make it difficult to fill the rear section, and its capacity is small, so it's easy to run it dry. With a helper pushing on the pedal I usually do about 6 stroke per side going around four or five times, and check the rear res level every 12 stokes when working on the rears. A full front res. is good for about double the number of pedal strokes.

Fill the res. to near the top of the opening, then wiggle it back and forth to fill the rear res., and check it frequently as described above to keep track of the fluid level, Don't let it get lower than at least a quarter-inch above the bottom.

When doing a brake fluid change I siphon the front res, but can't reach the rear due to the internal baffling, so I bleed the rear brakes until the rear res is about a quarter-inch above the bottom, then fill the res to nearly the top of the opening and proceed as described above.

I made the same mistake the first time I did a fluid flush over 25 years ago on my 190E 2.6 five speed, figured out the problem, and have proceeded as described above on all subsequent flushes without ever running the rear res. dry again.

Duke