GM started putting those charcoal canister filters on the brake boosters vacuum hose around 73 or so--they absorb oil and gas fumes from the crankcase and they keep the fumes from rotting the diaphram in the booster out prematurely...as noted,they do sometimes clog up and cut or reduce the vacuum to the booster sometimes.....
I had a truck I thought the booster was junk on,but it turned out to be someone used cheap gas line instead of genuine power brake hose (which has MUCH stiffer walls than gas line)--the fuel line is too squishy and collapses shut instead of letting the vacuum go thru..same thing happens when you use gas line on a PVC valve too--it often renders it inoperative because it collapses,and gets no vacuum!
I thought my booster was bad, so I replaced it thinking it was the culprit. Negative, ghostrider. It was that little filter. It had stopped up and was not allowing vacuum buildup.
