Lower hose should have had a spring in it to keep from collapsing under negative pressure. Did it have a spring in the hose? Also, I agree with Sixto, I think your thermostat should be rated for 87C, so it doesn't seem alarming at all that your temp is at 80C, which is well below MB standard operating temps of 87C - 105C.

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1990 190E 3.0L