My 86 109e 16v has been losing pedal pressure at times. Normally the brakes are firm, but occaisionally in traffic they go almost all the way to the floor. It only happens when I am holding the clutch in at the time (stop and go traffic mostly). It feels like only the front brakes are grabbing when this happens as the car nose dives hard. It doesn't happen all the time though, so far twice in the last week. As soon as i get moving fast again, the brakes are fine.

Any ideas?

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