"First things first.I have a main trap about 20 feet from the house between the house and the city sewage line as described below. That keeps me isolated from the fumes from the main city line right?"
Perhaps that running trap in your sewer line isolates you from the street main but you still have a open line to the sewerage in your home and in the house main which leaves you in the same situation as if the trap wasn't there. The fumes are just a hazardous to your family. I can not find you a repair part for your bell trap drain however I can give you a link where you can replace it
http://www.rhinounit.com/IPScorp/wt_...elltrpdrn.html
I would still recomend installing a regular cast iron floor drain. You would only have to break cement around the drain abd connecting drain pipe and you'd have to do that any way if you replaced with a bell trap drain.
"Another thing my main line exiting my house may not be low enough below the finished concrete slab to install flloor traps."
A regular floor drain won't eat up as much elevation as your bell trap, and I didn't say change out all the bell trap drains. Just the one that's broke and leaking sewer gas. Good luck, Tom