Fishslayer, Don't try using logic to understand the darned monovalve, it just works crummy at speed because of back pressure or something in the Benz "Climate Control System"
Seen this b4.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!