for a little while now I have noticed my fuel cap was getting harder to open and that a vacuum was being created in the tank. Things stated to come to a head when I noticed a dent in the fuel tank. At first I didn't relate the two issues together. When I would open the cap you could hear the tank whoosh for air and then the dent would disappear. On the way home from a ride with low fuel the bike actually started to cut out. So I pulled over so I could open the tank and let it breath and then went to the nearest service station.
So after looking up doctor Google I found the vent line and put compressed air into it with no luck. Tried from both top and bottom. So a mate and I pulled off the tank. We found that the breather line had actually been blocked solid by rust. The remedy was to spay inox-or WD40 and let it sit. We spayed it about 5 times in 45 mins and then it with compressed air.. The rust particles that came out looked like course sand.
I will metion that my bike has been ridden in the rain a few times and have been caught on the odd dirt but I don't think that is the root cause. Proably more from condensation. For the record it is a 17liter tank. Just thought I would post this incase someone else ends up with the same issue. As part of regular maintenance I will now keep spraying inox in there and giving it a blow out to prevent this happening again. For those that are not sure it is the hole with the orange rubber bit under the filler cap.
Keep the rubber side down
Thanks for the heads up will have to check