I bought a tube bender from Autozone to make up a fuel line on my boat - it was for the larger 5/16" metal tubing that I also bought from Autozone with the flared fittings already installed. The bender bent the tube alright but flattened it at the same time. Could have been operator error...

Maybe those cheap benders work OK on the smaller tubing but i have a suspicion i didn't spend enough money.