Our tub is leaking only when filled with water to use as a bath. It does not leak during any showers. We've replace the drain, cleaned out a clog and sealed up all areas we could think of that would leak. The tub began leaking again today when my daughter drew a bath. We do not overfill the tub and it is not leaking through the overfill plate. We can't figure it out. Please help with ideas/solutions. It is an old cast iron tub, but we cannot see any cracks.

Thank you!

A crack may be opening up when the tube if filled with the aditional water weight.

I would suggest that your remove the handles and escutcheon plate on you mixing valve. Turn the water on to fill the tub and inspect the mixing valve with a flashlight. You may find your leak is coming from the connection to the spout.