The monovalve contains a check valve to prevent hot coolant from backflowing into the heater core; it could leak, but what you may have is simply a heat-soaked climate control system from the hot engine and sitting in the sun.

I notice that when I take the car out of the cool garage, the a/c system does a great job, regardless of the outside temp. If the car has been sitting out in the sun, however, it has to work much harder. And if the car has been run and then parked in the sun, it's even slower to cool things off. Subsequent sitting in traffic is the final straw, it really struggles.

So your car may just be acting normally. However, you might consider pulling the monovalve apart sometime and see if it is crudded up inside to where the check valve isn't working correctly.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970