i think my master cylinder is shot?

If they're squishy, it sounds like you have air in the line. You will need to bleed them, but check to see that your rear wheel cylinders aren't bad. To do this, peel up the rubber grommet on the ends, and if there is fluid inside, they are bad. If it is dry inside, they are good.

Is the master a remanned? If so, it most likely uses the poly urethane (or whatever) seal, and those are crap. If you can find one from something older that uses a rubber seal, grab it. Also when I bought mine for front disc rear drum, the lid was warped just a little, hardly noticable, but enough to leak.

As for a vibration at highway speeds, since our cars don't have ABS, I would assume a warped rotor, but someone should chime in on this one.