The gas caps have little silicone flapper disks in them that vent one-way. When they start to deteriorate they get really sticky and ambient pressures aren't enough to break the seal. The pressure you're sensing when opening the cap may be air rushing into the gas tank.
Go to Joyner-USA.com They have the owner's manual for your year model Commando. The manual has the wiring diagram. If you have trouble getting and downloading the manual, PM me and I'll download and send you a PDF copy.
650commandoC2_owners_manual2008(1).pdf