Eagle diesel fuel air vents

[Gary LaBombard]

Hey guys,
Just a short question, the air vents next to my diesel fuel fill tube on each side of the bus are now open but I put a small screen in each of them to prevent mud bees etc. from clogging them.  My question, is there a (ONE WAY) type of valve normally put on these fittings near the fill tube cap so that pressure is kept on the fuel tanks?  

I am not sure if our diesel tanks require pressure to build up in the tanks like in our cars when you open the fill cap. The fitting in question is on the top right hand side of photo.  
Thanks ahead of time.

[blue_goose]

The tanks do not have any pressure in them.
Jack

[Gary LaBombard]

Thanks Jack, do you or anyone know what fitting is usually installed where I put that screen material?