What has happened is the big aftermarket companies like A1 Cardone figured out they could resell drum brake master cylinders as disk/drum masters by removing the residual valve on the rear port (to front brakes). It works good enough for most of the time. Now what happens as the pads wear is the piston starting position moves out. The extra fluid in the resevouir ensures there is always enough to keep the front lines and calipers filled. It also helps having cooler fluid available for multilple high speed stops.
But most of the time the smaller resevoir was enough. Cardone and the others don't really care that occassionally it may not be. If there is an accident and some expert points out this was a contributing factor then lawyers will get involved. Their lawyers will either settle or argue that you or the mechanic should have checked and topped off the master cylinder more frequently - whichever will cost them less.
So how do you know what I just wrote is correct or just BS?
One way is to go the factory information. The shop manuals show this and sometimes explain it. Even better in this case are the Chrysler Master Tech booklets about brakes and brake hydraulics. Thanks to the internet and some dedicated hobbyists, this is all available on the internet, at least for now. https://www.web.imperialclub.info/Repair/Lit/Master/
