I looked into having a non-MB power flushed recently as this vehicle has no drain plug in the pan and for a multitude of design reasons, is very difficult to do a fluid change on.
Valvoline fluid change center rep hands me a waiver to sign that absolves them from any liability. Power flushing has an unfortunate tendency to break things (dirt) loose. A speck of dirt can kill a tranny. I said thanks, but no thanks and went about the drudgery of doing it myself.
I don't know about your MB, but mine has a drain plug on both the pan and the converter. I can drain the entire system, at least I think I'm draining it all.
Stu Ritter of the Ritter-Easley list commented on your situation several months ago. According to Stu, a delay of about 2 or more seconds was indicative of a tired transmission.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.