I recently had a problem with losing my brakes, If I rev it up I get them back, but as soon as you use the brakes you lose them again, typically I would replace the vacuum pump, but how can you tell if the booster is losing the vacuum instead of the pump failing? Anyone know how much vacuum should be there?

 

Run your A/C and see if it starts blowing out of the defrost when you use the brakes heavily. That is a pretty good indicator of what your problem is.

Also-A word of advise- if you do have to but a vacume pump, bite the bullet and go get one from ford. It will cost you a few more dollars, but trust me, its worth it. There is something about a parts store vacume pump- They never last very long.