Disc brake Master cyl leak??

Well dad gave me a call tonight and said that after he got home from a short drive(brakes didnt feel right) He checked the MC and the reservoir closest to the firewall is empty. I believe the MC is fairly new or has been rebuilt, but the car sat for approx 2 years before Dad bought it. He said he checked the front discs and the rears and no leaks. I remember when we were doing some work under the dash that I could see a small drip on the rod that goes into the MC. Any ideas?? Its dads 67 cuda std disc brakes, so no booster. Im thinking maybe get away with replacing a couple seals?? I have never opened up a MC, so this is new to me/us.

Thanks
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