Here are the issues I had with my 67...low pedal hard stopping. Low pedal was resolved by adjusting the brake pedal pushrod. Hard stopping was caused by no front brakes. The brakes were not working because the rear res. on the MC was leaking on the inside of the firewall (interior of the car.) I've also seen the rubber lines swell due to age...causing a restriction the result being hard pedal, but no pressure at the caliper/slave cylinder.
I like to leave the brake pedal pushrod adjustment until most other causes are evaluated. I like your thinking on the master cylinder as the defective part. Most likely cause after it's determined that all his parts are in a stock configuration. Especially if there are no big puddles....
