There's a little BB in there that needs to jiggle around, so the crankcase can blow, but not suck. To test you should be able to blow through it one way, but not the other. Just get a new one, they're only a couple bucks.
99 uzj100, gx470
05 525EXC, 01 XT225, 07 TTR90, 07 CRF50