Even without the 'flame trap', the positive crankcase ventillation system can become clogged with hard deposits. The hoses themselves can become restricted, or various ports and orifices can become clogged or the oil vapor separator on the side of the block can become clogged. Often, the hoses, when clogged and old just need to be replaced. (BTW - running the engine with synthetic oil will help prevent clogging of the PCV system because the synthetic oil has a much higher vapor point so that you do not get as much oil vapor condensing and hardening in the system. And what vapor does exist does not coke as easily to form the hard deposits.)

The 'jiggle test' is to loosen but do not remove the oil filler cap while the engine is running. If there is negative pressure in the crankcase, it will be sucked down to the valve cover and will sit still. If there is positive pressure in the crankcase, the cap will dance or 'jiggle' on top of the valve cover as the pressure escapes.