Let me share an odd experience of mine that may be related to your problem.
A number of years ago I headed to the office early one morning in my '84 240D(same chassis as yours). My office is 90 miles from the house and it was still dark. I was about 15 miles from the house and thought the engine was losing power. I slowed to get on the uphill ramp to the highway and the car wouldn't accelerate up the ramp.
I stopped, got out and started walking around the car. I immediately noticed the left rear brake rotor was RED HOT. It looked like a red neon sign. I walked to the other side and that rotor was glowing as well.
I was lucky enough to be where I had a car phone signal, and called the wife to come get me. By the time she got there, I had found and cured the problem. I got an open end out of the toolbox and put it on the master cylinder fitting for the rear brakes. I put a rag over it and cracked the fitting open. I felt fluid shoot out over my hand. There was extreme pressure there, and noone had their foot on the brake. I bought a master / cylinder that day and pitched it in the trunk. I went a long time after that with no more problems and some time later when doing a brake job, I put on the new master cylinder, after all I had already paid for it and it was there.
I can only assume that somehow the master cylinder did this, but I'll never know for sure.
BTW, with that said, don't overlook the fact that the emergency brake consists of shoes inside the rear, outer rotors that are actuated by a mechanical mechanism.
Best of luck,