weak brakes

Fox brakes arn't that great, but they are not THAT bad either. You should have pin pointed the problem first before throwing parts at it. It could be anything and everything who knows. Brake fluid gets contanimated because it is hydroscopic which means it absorbs water, right out of the air. That's why it gets black when it is clear new. Especailly with old lines and seals, air can get in there. The smallest amound of air will cause the brakes to not grab at all.

Firs of all check that the caliper slide pins are moving freely. Then check that the rear drums are properly adjusted. Suck the fluid out of the master cylinder as much as you can and add new fluid, then leaving the cap off open all the bleeders and let it flush out until there is all new clear brake fluid comming out. One more thing, are the rotors and or drums glazed?

You just figured this out? WOW!

Does your car hold on a hill with the E brake? If not, you need to manually adjust up the rear shoes.

Really the next step is to start measuring. You need to measure the amount of vacuum at the booster check valve and after it. Measure the fluid pressure at each wheel on the car. You may want to stop by like a Midas or something to give you a free safety inspection if you cannot diagnose the issue yourself.