Brake Failure

Bad master cylinder or leak that you haven't found. Air generally won't get in the system from a rust pin hole leak, but the fluid will escape.

Yes.

If it's a 4-wheel drum setup, you could have also lost a wheel cylinder. Is there any brake fluid left in the master cylinder or has it disappeared? It could also be a broken brake line. If the master cyl is empty, fill it with some fluid, stomp on the brake, then aim for the wet spot on the floor.

Cheers,

It doesn't seem to lose fluid when I stomp on the pedal though. It's always had a very slow leak but nothing serious. I would have to top it off every few months but the bowl was never less than half full. Also for the last 2 years, when stopped at a light the pedal would slowly lose pressure and would require a pump to bring back the firmness. Sign of master cylinder failure?

Master cylinder or a wheel cylinder.

Before you buy a new master cylinder I would visually inspect both the front disks calipers and rear drum wheel cylinders for leaks. My guess would be that you've got a caliper piston seal/boot leak. You will have to remove the calipers to verify this one way or the other as you can't tell from just looking on the outside.

My2cents

Sounds like it's the master cylinder then. However, if you're losing fluid, you still want to find out where it's going. It may have just been running down the firewall behind the master cyl or you may have a leaking line or wheel cylinder in addition to a bad master cylinder. Don't forget to periodically (every 2 years or so) bleed ALL the brake fluid out and replace it with fresh fluid.

Cheers,