Another brake related issue

I tried searching to see if I could pull up a similar thread, but to no avail.

Here's my issue:

When I hit the pedal its firm, but after a few seconds it starts to soften and the brake become useless. The pressure will come back after a pump or two on the pedal but still goes away rather quickly. I've heard that it might be that I need a new master cylinder.

Has anyone had the same problem and is there a quick solution?

Thoughts/suggestions?