My '88 has been sitting for about 4 years. Couple days ago I got it out...decided to get'er road worthy again. There was no brake fluid in resivoir, so I filled it up but pedal would never pump up. Maybe it just leaked out over the last 4 years?...maybe they need to be bled?
When I had it out it ran good, but under wot it would miss and sputter (4.9L). Maybe just old gas...?

 

It has most likely got a steel brake line rusted out and leaking or a wheel cylinder dried out and leaking. Even if not if there was low brake fluid (it doesn't evaporate so likely a leak) and you added some if there's air in the line you'll never get a solid pedal.
But brake fluid doesn't just go missing, you'll have to find the leak.
As for the miss, yeah 4 year old gas could do that.