If there is a tear in the rubber diaphragm within the monovalve assembly, then you will get hot coolant circulating in the heater core when you want a/c.

Parallel flow condenser will work **substantiallly** better with r134a than the original tube condenser which was designed for r12. The original r12 condensers the w123 and w126 chassis Mercedes came with will work with r134a but will have lackluster performance in hot weather. You can get universal parallel flow condensers, but you will also need to know how to make custom a/c hoses, which a good a/c shop can do. Cabin temps will be in the 40's with a parallel flow condenser and r134a.

https://www.youtube.com/watch?v=M5lvHxCVxWg