I am slowly restoring a 74 Bus that went through a salt water bath up to
the mid height of the van.  One of the things I want to replace is the gas
tank venting system.  The lines are steel with some type of check valve in
them.  I want to replace it with plastic, but I can't find any information
on exactly what the check valves are or how they are supposed to work.  If
anyone knows about this system or can point me to some good information I
would really appreciate it.

Ken