Vintage Double Hose

Dacor lowboy 1/2 tapered thread J valve. When valve is opened to dive it leaks a trickle of bubbles out past the knob. Will shut off fine with no air loss.
Any suggestions?

Thanks

It is leaking from the stem seal. It can be replaced with out emptying the tank. With the valve closed remove th knob, remove the bonnet nut and stem, replace the white teflon seal on the stem, reassemble.