Yes. The check valve is a one way deal.
Found this tidbit on the JAZ Products site:
Can your Vented Caps be used as a stand-alone vent for the fuel cell?
No. Our vented caps are not a sufficient standalone vent for the fuel cells. These have a minimal vent that allows for displacement of fuel. The fuel cell requires a completely open vent fitting installed. If tank is not properly vented, fuel starvation may occur.
And this one on racingarticles.com:
Inadequate fuel cell venting.
If the fuel cell vent is too small the fuel system can malfunction. In extreme cases, inadequate ventilation can cause permanent damage to the system. As the fuel pump draws fuel from the cell it needs to be replaced by air. If the vent on the cell is too small the pump will try to draw the fuel from the cell faster than the air replaces it. This can create a vacuum in the cell, distort its shape, and starve the pump and engine of the fuel it requires.
This was found on stockcarracing.com:
The rollover valve allows air to vent in as the fuel is pumped out. But on the occasion when the car is in an attitude where the fuel would run out, this valve shuts by its internal weight sealing the fuel inside. The last thing you want is to be trying to get out of an overturned car while a small stream of fuel is busy soaking your fire suit.
Steve
