How can I locate the drain plumbing in my old house?
On homes that have sewer under the slab I have rented a tracer and purchased a tracker ball I flush the ball (I have attached fishing line to the ball to controll how far it goes) I then use the tracker to figure which way the pipes go. Once I get the general direction I will release some line and flush again, tracing where the ball goes until it quits moving. I have found septic tanks using this method that were more than 100' away from where it was shown on the plot plan. Several times I was able to retrieve the ball but I have also lost a few they are designed to be flushed I just hate dumping 20-30$ down the drain (pun intended). My local equipment rental place rents the receiver but you have to purchase the balls and once activated some only have several hours useful life. I have cleaned a few up and opened them and pulled the battery and was able to reuse but some are sealed so well it damages them beyond use. This is the method I have used to figure out where the plumbing goes and also find a tank that was not where the plans showed. At the age of your home this method could save a lot of time and expense as you will be able to cut a trench above the pipe to allow for replacement as it sounds like your pipe may be leaking, the only way to stop the leaks cost effectively is to replace the old cast iron pipe with plastic.