When was the last time you adjusted the valves?

 

PCV could be malfunctioning.

 

Maintaining vacuum on my '76 is for some reason a constant battle. Soon as I find and fix one leak, it develops another. My latest one was a vacuum advance diaphragm gone bad. Found it the old fashioned way: connected a hose and sucked, but my tongue didn't stay 'stuck'. Replaced with good used one and I'm back in business. However, my car has never measured more than 15 vacuum. THis despite having replace ALL seals and gaskets except the rear seal over the past 3-4 years.

Your PCV valve could be bad. Yes. They are cheap and relatively easy to replace. Just replace it. Your leak could also be bad hoses, bad (loose) conections, bad components -- in fact, it could be about 20 different things. I would suggest that you block off the vacuum to the heater/duct assembly (that white plastic bottle on your passenger side under the hood). That's a common place for leaks. Then, one by one, squeeze off various components with vice grips or hemostats and see if you can isolate a leak.

You could also be leaking air through a rotten accordion hose from your AFM to the throttle body (had one of these on my car), or through a leaking AAR (had that, too). The factory service manual can help a lot in tracking this stuff down in a logical manner. FWIW, once you DO get the vacuum tight again, it's amazing how well and smoothly these engines run.

 

Check your pcv valve (bottom of the intake with a hose that runs to the timing cover). Good chance it's stuck open. Also check your valve cover for leaks and any and all vacuum lines coming off the intake.