Underneath my bathroom sink, there is an alternate pipe running from the drainage pipe to the wall. The main pipe is copper and T's off at an upward angle (also copper), then this pipe become a rubber hose attached with 2 hose clamps (one at each end) until it attaches to PVC at the wall. The one end is leaking every time I run the water. My question is:

1. What is this pipe used for?
2. How do I stop this from leaking?

The sink drains fine, very fast.

Thanks

1. What is this pipe used for? My guess ius that it's a AC condensate line run down from your air handler.

2. How do I stop this from leaking?
Tighten the hose clamps on the hose.
Good luck, Tom