Look under the dash and under the carpet all on the driver's side. If you see or feel fluid your master cyl is leaking. If it's power it might be leaking into the booster.
Google " Chrysler master tech video" and the subject at hand "brakes"
The pamphlet that goes with the filmstrip is here. However it focuses on the drum system https://mymopar.com/browse-mtsc-by-model-year-1967/
You will also want to be familiar with the disk brake system, which is also covered in the 1967 MTSC.