I replaced the two check valves on the attached 380SL vacuum diagram. #17 check valve was open both ways and looked like the original part. #26 was missing. Ahem! You have to take the air filter housing off to get at #26. #17 is just underneath the air filter housing at the front and can be extracted and replaced with a long set of forceps or needle nose pliers.
Fast idle has dropped from 1400 rpm to 1050-1100 rpm. Still not to spec but better. Way better.
Transmission flare has disappeared as well. Need to test drive it again to double confirm but was behaving well yesterday. Modulator valve was replaced at the same time.
I could not understand the configuration of part #26 from the diagram on EPC or the shop manual and had to try it both ways round!
Still have to pull the tranny to fix the seals but this is progress at last.