I had that happen to me before I upgraded my brakes on my 85. I actually drove it for 2 months like that, i did have some, but very little brake pressure and like you I had to pump the pedal three times to get any pressure. So I just anticipated braking for 2 months until I swapped everything out. Im a lazy b*std . Aside from my brakes needing to be flushed and bled, some gear oil and grease had seeped from one side of my front axle and had gummed up the calipers so bad they basically didnt work worth 
on one side. After replacing the rotors, calipers/pads, and master cylinder, flushing and bleeding the system, they worked great. I dont think it was anything more than several large bubles of air in the system on top of the greased filled caliper though. I am nearly possitive that the master cylinder was fine but I replaced it with a V6 one anyway since I was upgrading.
Try bleeding the brakes and if that doesnt work check the master cylinder and the booster. Should do the trick but if not let us know. while your at it making sure the calipers arent gunked up is worth examining too
tomorrow i need to rig up an extended break like for the rear connector at the T on the axle. i know how to do it its just a matter of getting it done. then im going to bleed the system and test it out. hopefully that works. the calipers ive never cleaned in my life...what do i do with that...take the pads off and spray break cleaner in them or what??
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