How do you repair a leaking return pipe?

Where ever it is leaking, there is some form of gasket. The connection may just need to be tightened or the gasket replaced. The gaskets are different depending on where they are.

Improved answer. -If it's an old grey plastic or steel pipe, it's almost impossible to seal leaks at either end. Take it off and go to Home Depot and buy a new SS braided rubber hose for less than $5. - These have Excellent gaskets at each end. -Sometimes it's just not worth it to fool around with old junk !