[DeTomaso] vent line weirdness

[Christopher Kimball]

First, thanks to David from Maryland who solved my seatbelt dilemma (at a great price, too!).. Thanks Dave.
 
Now, I need some opinions about a strange occurrence.
 
In preparation for the Vegas trip, I drove the Pantera 200 miles yesterday.  The weather was so weird, within two hours I drove in 75 degree, sunny weather, then through driving rain and finally through a snow-filled blizzard.  I am not kidding!  The car had no water leaks, the heater and air conditioner worked great, so I was feeling happy about things.
 
While driving, however, I heard a "thunk" from directly behind me.  I figured something must have fallen over in the trunk.  At the next gas stop I checked, but no sign of problems there.
 
When I unscrewed the gas cap, however, a loud hissing ensued and I heard that loud "thunk" again.  I realized the vent tube must be clogged and the first thunk was the sound of my gas tank collapsing as the fuel pump sucked out the gas!  The other thunk was the tank popping back into its (hopefully) original shape.  (I checked when I got home and I either got 22 miles per gallon, or my tank is now several gallons smaller...)
 
I checked the vent hose which I had run to the air filter, and it was clear all the way almost to the tank.  At that point, it was plugged.
 
I used a bicycle pump hooked to the hose to force air through it (with the gas cap removed, of course) and that freed it up so it was working correctly again.
 
The only thing I can figure is the little rollover-protection valve I installed in the line must have failed.  It was installed to allow air to go into the tank, but in the event of a rollover stop fuel from flowing out.  I know it is installed correctly and in the correct direction.  I haven't had a chance yet to remove and inspect it.
 
Has anyone heard of those little rollover-protection valves failing in this way?
 
Thanks,
 
 
Chris