I just completed a citric flush on my 75 300d OM617 (w115 car) and flushed and backflushed many times and also used air to blow out remaining water. Yesterday, I noticed when bleeding the system that I thought I detected a rush of rust colored water and wondered since I did not open block drain is it possible to trap rusty water somehow in spite of flushing/backflushing? I must have flushed about a dozen times until water ran clear. Someway that water can be trapped in the circuit near the block drain? Thanks!