Hello,
A leak has developed beneath my toilet cistern. Years ago, my father
installed one of those in-line stop-taps on the pip supplying the
cistern. If I turn the tap off, the leak stops, with just a slight
weep until it ceases altogether. If I open the tap, the leak starts
again.

The washer beneath the cistern is dry. There is no leak, with the
stop-tap turned off and the cistern filled from a jug. The leak
appears to be from the thread joint between the float valve and the
stop-tap.

Coincidentally, it seemed to start when the ball was adjusted to stop
the cistern overfilling slightly, and running through the overflow.

Is there anything I can do to stop the leak? PTFE tape? Is there any
reason why it should start leaking after all these years?

Thanks,
John