I have found there to be a discrepency between right and wrong at the big chain parts stores. What enthusiast isnt tired of answering pointless, mundane questions that do not matter?...Yeah, I need a master cylinder for a 68 Dart with manual disc....." Uh, what motor? Manual trans? Power steering? 4 Door?... NONE OF THIS STUFF MATTERS ! The majority of parts counter guys are drones that know less about cars than my mother. They were trained to use a computer and to help old ladies install wiper blades on their cars in the store parking lot. MUCHO respect to the pitiful few competent parts guys still in the business. They are smart enough to bypass the unimportant questions when doing a parts search. Okay, on to your situation. Are you sure that either of those new MCs are correct? On a disc car, power or not, isn't the front brake reservior supposed to be bigger? It seems that all the dual circuit disc MCs I remember had the big reservior nearest the firewall.