Thanks for your responses, guys - The brakes seem to be fine at idle when moving in a straight line (forward or backward); the only time I notice a nearly complete loss of braking is when backing into a parallel parking spot, when the front wheels are turned nearly to a left lock - the brakes will then remain faded until I pull back out and give it a bit of fuel. This seems to be consistent with what you were saying, desmondo - and the vacuum idea seems a good bet too, d.l. - I just can't understand why it only seems to be brought on in a tight turn situation. Any relationship to vacuum assist in the power steering, maybe?
1980 300SD (The Green Machine!)
180D Ponton (paid $100 Cdn - sold $200!)