Ford E350 braking problem

Hi all,
I have a 2000 Ford Van 350. Today the pedal went to the floor while driving/braking. After a few seconds the pedal will come back up and
will brake for a second or two and then drop back to the floor. Does this most likely sound like the Master cylinder? How hard is it to change it over (for someone who is machanicallly inclined? Is it as easy as removing the two bolts holding it on and just switching over the brake lines, and then bleeding them?
Thanks for any advice! Bret
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