I always siphon the old fluid out of the reservoir, but you can only get the front/clutch part of it. Once you've siphoned out all you can fill the res. with fresh fluid

Now you should be able to see the rear reservoir level from the color contrast. The rear level on my 201 can be seen at the very rear of the res. from the right side. I think 124s are the same. It's real easy to bleed the rear dry and get air in the rear system because a lot of guys don't know how to rear the rear res. level.

With the front res. full of fresh fluid, bleed the rear down to about 1/4" above the bottom. Then rock the res. sideways so the clear fluid in the front gets over the baffle that separates the two, then top off the fluid, rock the res. again and keep bleeding the rear. Repeat until the fluid at the bleeder valves and res. is clean.

Once the rear res. fluid is clean, do the front.

Don't forget to do the clutch if it's a manual trans.

Duke