The 2 aft ports are connected together, i.e. the MC serves as a "T" for the front brake tubing, so no surprise they both flowed. I don't know why your front port (rear brakes) didn't flow much. I replaced the MC on both my W123's and saw a healthy flow from both F & R systems. I "bench" bleed w/ MC on the car, using the pedal to push. If you were truly doing it on the bench, I suspect you weren't able to push hard enough to move the 2nd piston. There is a spring between the 2. I used plastic screw-in port fittings w/ thin clear tubing to return to the reservoir. Those came w/ the MC or I had from prior work. If your tube is larger it might provide less restriction, but doubt that matters since I have used steel 3/16" tubing on other cars w/ no issues.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans