If I remember correctly, there is a small plastic filter basket within the mono valve housing. I believe you have to remove the mono valve to look down inside to see it. I'm thinking If you flushed your coolant system recently (recent radiator replacement) this filter may have enough sediment build up to cause your issue.

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BENZSTER

W123, '81 240D, Pastel Grey/Blue Tex
W126, '88 300 SEL, Artic White/Grey Leather