After my trip to Grand Canyon and Bryce with my daughter, I went on a mission tearing things apart in Sharka to fix the gas fume problem. In the heat of the summer and with the huge elevation changes, the fumes were really really bad. Happily, not so bad when driving but if we stopped for even a minute, they would start to creep into the cabin.
So I started investigating once we were home and Sharka could be on jack stands for a few weeks. I started by popping a hose off the charcoal canister (evap expansion tank) and pressurizing the fuel tank manually with some compressed shop air. Not a whole lot of pressure, just a few puffs from the air gun so that I could hear the tank expand. Then I held the hose to my hand (or cheek, lol) to feel the pressure escape.
Nothing. No pressure ever escaped.
Suspecting I had a lot of crud built up in the hardline between the evap tank and the fuel tank, I opened up both ends of that hardline (the other is right near the tank) and powered a huge amount of pressure through it with my air compressor in both directions.
This hard line cleaning didnt do much for me and almost nothing was jammed in that line. A few specks of dirt came out of either end, but air flowed freely through the line all the time.
The only other things between the tank and the charcoal canister are the rollover valve and that little white check valve. As it was the easiest next step, I swapped a piece of straight hose in place of the check valve hose and reconnected the hard line.
I pressurized the tank as before. The tank expanded audibly. Then I released the air gun and
THE TANK RELEASED PRESSURE.
So the failure mode of that little valve WHICH IS SUPPOSED TO RELEASE PRESSURE is to not ever release anything and become blocked. Cleaning it lasts for a few months to a year and then it is back to its previous dangerous behavior.
I deleted the valve permanently and used a piece of silicone hose instead. And I did this months ago back in August 2018. Ive been sitting on this post since then, driving around and making sure the pressure didnt return.
The fuel tank fumes and overpressure condition never returned.
CAUTION! This may not be a good solution. Id suggest sourcing a new valve from Mazda and replacing the old one.