Leak in return pipe.

Is this a wet return (down near the floor) or a dry return (an extension of your Steam Main, after the last Radiator)? How big of a leak is it? If small, you can try to clean it really well and apply some JB Weld and see if it holds until the weather breaks. If it's in the middle of a section of pipe, It obviously should be replaced as soon as the weather allows. If it is at a joint, depending on how bad it is, JB Weld may hold, if at a Flange joint, there re gaskets sandwiched inbetween the flanges that need to be replaced.

You can always find the clamp type pipe repair kit, with a rubber gasket, which will close up a leak. In both cases, the pipe surface around the repair must be cleaned/sanded thoroughly. If the JB weld does not work, then all traces of the epoxy must be removed to make the clamp type repair work, and that can be difficult.--NBC