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....
I agree with Jim. The pedal pushrod was right when the car was delivered.
This is a forty year old car. There is nothing more important to safety than being able to stop, unless it's being able to steer. The entire brake system should be gone through, renewing all hoses, cylinders, tubes and linings before putting the car on the road. This is one repair that should be shot-gunned with new parts rather than diagnosed one cheap part at a time.
It's funny how people start restoration with the engine. You need to be able to stop and steer first. That is where repairs should begin.
