Mystery brakes & acceleration issue
 

Hi. Car is a W123 body 1982 Euro version 200 with a 2.0 gasoline engine.
It seems I am only occaisionally having a strange issue where I am driving at highway speeds and out of nowhere the engine starts to lose power having to downshift to 3rd just to get back to 35-40mph. I just had the carb and fuel pump cleaned and 90% of the time the car runs with full power and smoothly. The coincidence or not is when this happens the last 3 times I stop the car for a few minutes and when I get going again my brake pedal gets softer than normal. (After driving just a short distance the pedal goes back to normal) The braking issue only happened when the accelration issue happens. I am not losing brake fluid and 99% of the time my brakes are normal. All I can think is this might be an electrical issue if that's possible on an old W123.
Any suggestions or fixes are very welcome.

Assuming it has a vacuum brake booster it sounds like brake fluid is being intermittently drawn into the booster and then into the engine, especially if there is no evidence of an external brake fluid leak. So I'd focus on checking function of the master cylinder and either rebuilding or replacing it with a new or rebuilt unit.

Also check that the booster holds vacuum.

Has the car had regular brake fluid flushes?

Duke

Duke the OP says no brake fluid loss...was wondering about the check valve in the booster line hanging up? Probably worth pulling the MC loose but leaving the lines attached to leak at the back of the MC for leakage.

Was wondering about a brake hanging up mimicking power loss. Do any of the wheels get warm when this happens?

1) The simple fault: The check valve in the vacuum line between intake manifold and brake booster is sticking in the open position.

2) The more complex fault: The vacuum control valve inside the brake booster is sticking open.

Oops, I read it as losing brake fluid. Sorry.

Duke

It almost sounds like a brake problem could be causing the power loss. Perhaps a caliper is sticking making it hard to accelerate. I can't imagine that the brakes could self apply as you were going along. When you stopped along the road did you smell overheated brake pads? An overheating brake can heat soak and boil the fluid making braking poor until the air cools the fluid in that brake and it can condense again. I think I would jack each wheel up and turn each wheel looking for abnormal drag. Wheels with drum brakes will turn easily and disk brakes will drag on the pads but will be fairly easy to turn by hand. Of course a stuck or seized caliper would only apply to disk brakes.