/diy/ - Do It Yourself

>buy 1940s house
>previous owner re-did the old plumbing
>decide to replace the guts in the terlet because it doesn't flush well
>turn off the water supply line
>water starts leaking downstairs immediately after
>have a hunch that turning the supply line back on will stop the leak
>finish the job
>turn on the supply line back on
>the leak stops
Do any anons have any idea why turning off a water supply line would cause a leak, but having the line open, but not flowing would not cause a leak?
I have a feeling I'm going to have to replace all the supply lines anyways, but I'm curious.