I am helping a fellow AACA member with his 25 Chyrsler that has external band hydraulic brakes. I am not sure that the master cylinder is an original type or not, but we can not get the bands to adjust tight enough without binding the drums when the pedal is released. Even when adjusted this tight the pedal goes almost to the floor and will not stop the car. The wheel cylinders have all been rebuilt and the master cylinder was purchased as a NOS item from NAPA. (I wonder how many years that it sat on the shelf) There are no leaks and they have been bled so I do not think that air is the problem. Any ideas,tips and experience with these type brakes are welcome to my Email addr.
dmm671@suddenlink.net

Dale, from what I am reading there must be air in the lines or the "new" master cylinder you got from NAPA must be too small ( not enough volume). I am amazed they had a part for a 80+ year old car on the shelf. I would study the movement of the brake arms while someone depresses the pedal in the car. These are the exposed levers that transfer the push of the wheel cylinder to the shoes. If these do not show movement you are not getting fluid to the wheel cylinders or air in the system is taking up the stroke. Note: these cars had a reservoir separate from the master cylinder that the driver had to manually pump to get fluid to the master cylinder. If the valve in this reservoir/pump is not snugged down the fluid will simply go from the master cylinder to the reservoir and not to the wheels. The brake bands should be set at .010" to no more than .015" arround the drum.

Here is an explaination from a '28 series "72" owners manual.

Thank you for the info. Presently I am thinking that maybe the
resevior tank valve is not sealing when there is presure when stepping on the brake pedal. There is some movement of the wheel cylinder arms but not enough to clamp the bands tightly.
I think that the master cylinder is not the original type but
a universal replacement but I believe it should have enough volume.