Lost my brakes, pedal goes all the way to the floor, I added fluid and pumped up the brake pedal. But this only lasted about 30 seconds then no brakes again. I did not see any fluid on the ground. Is this the master cylinder?

You said you added fluid and pumped the brake....after it went bad again, is the fluid low?

If it is low again then clearly there is a leak. It could be inside the brake drums from a wheel cylinder or possibly at the seal on the master cylinder.

Just double check on all the lines and make sure none of them are wet. Loosen the master and pull it off the power booster and see if there is fluid in there.

Pump up the master and then lightly hold foot on pedal, if it slowly goes down, it is a bad master cylinder.

Master cylinders are pretty tough. It wouldn't surprise me if a rear wheel cylinder is leaking. Unless you remove the rear drums, you might miss the leaking fluid. To help prevent brake problems in the future, change the brake fluid every 2 years.