If it's really gummed-up, might soak overnight in a cleaner/degreaser solution. But the main thing is that the spring-loaded valve inside will rattle, as noted, plus that vacuum will flow through it. The valve has calibrated orifices in it that pass the crankcase vapors into the carb/engine for recycling. But the main criteria for replacement was usually "the rattle" when shaken, back then.
I've had good results with spraying carb cleaner into the valve, with the carb-end blocked-off, then closing the other end after filling it with carb cleaner, then shaking for a bit. Repeat until clear liguid drains out of it. You can easily tell when things are free inside the valve, by how easily "the rattle" happens.
DO check the port on the carb to see if it's carboned-up, too. Which can be done with a flat-blade screwdriver. If it's very restricted, taking the carb off to manually do it might be needed. Being careful not to damage the carb base gasket, so you can possibly re-use it.
Enjoy!
CBODY67
