A neighbor asked me to look at her toilet and before I start yanking I thought I'd ask some advice. There is a crack on the very bottom of the overflow tube. The crack is at the base where it is somehow attached to the main body of the flapper valve mechanism. Does this tube have threads and can I unscrew it? Is it a friction fit? I'd like a closer look but there is no visible way to detach the flapper valve mechanism (not the flapper valve itself, but the sealing gasket part of it) from the bottom of the tank. The make and model of this toilet is a mystery. It's old, though. Any ideas on how I could remove the cracked overflow tube without doing more damage? Thanks!