1965 Dodge Dart Charger
The single-pot disc brake master cylinder has an extension between the top of the casting and the lid. It's because disc brakes use more fluid volume than drum brakes (the fluid level drops as the pads wear and the caliper pistons gradually move inward towards the rotors). Same reason dual-chamber master cylinders for disc brakes have a larger reservoir for the discs than for the drums. Part numbers for the disc brake master cylinder with reservoir for '65 and '66 A-body are 2660 791 w/power disc brakes or 2660 786 w/nonpower disc brakes.
Here's the page from the '66 FPC:
The single-pot disc brake master cylinder has an extension between the top of the casting and the lid. It's because disc brakes use more fluid volume than drum brakes (the fluid level drops as the pads wear and the caliper pistons gradually move inward towards the rotors). Same reason dual-chamber master cylinders for disc brakes have a larger reservoir for the discs than for the drums. Part numbers for the disc brake master cylinder with reservoir for '65 and '66 A-body are 2660 791 w/power disc brakes or 2660 786 w/nonpower disc brakes.
Here's the page from the '66 FPC: