Petcock may be clogged with rust. Notice in upper photo I posted above, two filters on inlet tubes of petcock was missing on my bike. You'll want to replace those with new ones along with small in-line filter
inside fuel-hose just before carbs. Open end faces tank and closed-end aims towards carbs.
Once you get tank as clean of rust as you can, you'll want to coat it to prevent rust from coming back. Many products on market for that. I like POR15 version myself.
https://www.por15.com/POR-15-Fuel-Tank-Sealer. Many people have had good luck with plain 'ol fibreglass epoxy resin diluted slightly with acetone.