If you are experiencing minor coolant loss on your older model that has the cap on the radiator, it may because you do not have the correct cap. Not all caps are created equal as Kent will point out in the video below:
Product Details
This is the type of radiator that will require this cap.
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Cooling System Leak Detection Radiator Cap Conversion for Brake Bleeder Tank
Have you ever had a hard time finally a very small coolant leak? Do you find yourself adding small amounts of coolant but can not see any leak? Is there a funny smell coming from inside your Benz when you run the heater?
201 124 Chassis All Models Coolant Expansion Tank Cap
Brand New Reutter Expansion Tank Cap marked 140. This fits all 201 chassis 190E and 190D from 1984 to 1993 and all 124 chassis sedans, coupes and wagons from 1986 to 1995 both gas and diesel. A worn expansion cap is the number one reason for very gradual coolant loss.