bad brakes

[ken]

My brake booster failed about a month back. So i bought a rebuild kit and overhauled it again. It was about 5 or 6 years since i did it last. But after the rebuild it did the same thing. I had no boost about every 5th time i hit the pedal. My master cylinder was leaking also. So i got a new master and i new booster from saturn. The booster is the smaller new style. I also pulled the hubs and the shoes and wheel cylinders looked good. I've had the new master and booster on for about 600 miles now. And yesterday while driving in heavy traffic the booster decides not to work. After franticly hitting the peadal 3 times it worked fine and stopped the truck while skidding the rear tires. It's worked good since that incident, but i'm gun shy of it now. I drained the tanks and didn't get much water. The air pressure holds good at 100psi. I'm using DOT5 from a 5 gallon can that i got from surplus. And it's dated for it's next inspection in 2009. But i don't think BF goes bad? I also flused all the lines when i swapped/bled the brakes. And each bleeder had clean purple fluid.
Does anybody know or have anything on these new style boosters? Or mabye i'm over looking something else? Mabye something i missed that might carry this problem over?