Ok getting details out of the way first.

74 F100, 2wd, 302, manual steering, power brakes.

I picked it up a few days ago and it looks like the guy did a number on it. Everything was nice with just a bit of rust and now that I've driven it a few days there are a ton of problems coming out of the closet.

First off and the most major one is the brakes. Truck was bought knowing that the discs were going to need replaced. However that does not appear to be the issue. When coming to a stop under moderate speed the brakes are locking up and the rear starts to slide, so I'm assuming it's the rears locking. At first I thought it was the bias-ply tires not having enough stick, but last night I found brake fluid seeping down the front of the power brake booster. When I removed the top of the master cylinder the front resivoir(rear brakes) was empty almost empty. I'm assuming its just a bad seal between the master cylinder and the brake booster since the lines themselves are dry. I've read in a few places though about leaks at the booster itself and they recomend replacement. Any thoughts?

There are a ton of other things that need done but I'm most concerned with stopping since I'm living in Phoenix and no one knows how to drive here.

 

Sounds like you need a master cylnder. It can leak and go between the cylnder and booster. Pull the rear wheels and drums. If they're locking up, they're either over adjusted, hardwear missing or broken, axle seal leaking and have brake shoes contanimated, that will make a wheel lock up. Also get a good areosol brake cleaner, clean everything, even the backing plate. See if the wheel cylnders are leaking.
If the brake shoes have any type of contanimation on them replace, and a new hardwear kit wouldn't be a bad idea.
Going through the same thing with my 79 play toy.
Bob