Fuel Fill and Venting

I am now driving my Saxon for 100 to 200 miles at a time and I have discovered that on the bottom half of the tank capacity it sucks the tank with the fuel pump enough to start to collapse the tank with a lot of pressure. I am looking for a source for the right size cap to replace the current one which is technically vented but doesn’t seem to let preasure in only out. From the assembly manual was I don’t see any vent system. I have the 19 gallon CR tank. Has anybody else experience this and found a specific cure?
Thanks

Had exactly the same problem. Suspect there is a more elegant and technically correct solution but i just drilled a 1/16" hole in the cap. Never have any excess fumes in the garage.

Jerry & Lisa Mills wrote:Had exactly the same problem. Suspect there is a more elegant and technically correct solution but i just drilled a 1/16" hole in the cap. Never have any excess fumes in the garage.

That's pretty much what a factory vented cap is. Just a sealed cap with a small hole in it.