Gas tank venting issues [1978 GS400C]

[ItsBork]

Where does the gas tank vent on the 400C? There doesn't seem to be any holes in the cap and ever since I added a second layer of Redkote I'm having ventilation issues. I'm guessing the tank vents somewhere else and I've sealed it up with the tank liner.

I assume the easiest solution would be to buy an aftermarket vented cap but I wanted to see if my theory was right first because I'd like to maintain the stock cap if I can.

[Suzukian]

I'd see if if I could find a tube to slip on the back side of the fuel cap to see if it vents there. Spray it with Carb Cleaner, to loosen anything there, and afterwards try blowing into the bottom of the cap to see if you can get air flow.