I have a 1974 nova. A few nights ago the brake pedal started sinking to the floor. But when I pumped up the brakes three times the pedal returned to normal. I thought it was a vacuum leak so I replaced the booster fitting and inspected the vacuum hose and grommet. All where rubber parts where fine. I am going to pull the wheels off and inspect the brakes for leaks. If no leaks I will bleed the brakes. Last resort is the master cylinder. I am leaving towards the master cylinder. If no leaks or air in the lines then it's the master cylinder? Please any information will be helpful.