[dkmaddux]

When it rains real hard, pressure builds up in our master bathroom toilet and leaks at wax ring... why?

[speedball1]

when it rains real hard, pressure builds up in our master bathroom toilet and leaks at wax ring....why?

You are discharging rain water runoff into your sewer system. Septic tank or city sewer? I'm assuming city sewer because nobodys stupid enough to connect to a septic tank. In my area it's against the law to put anything in the city sewer except solid waste and gray water.
You may have a partial blockage on down the line but the heavy rain has overloaded your system and it's backing up past the toilet.
The ideal solution would be to locate and disconnect the gutters from your drainage system and discharge them some where else.
If that's not a option I would snake the house cleanout to the street and wait for the next heavy rain storm. Good luck, tom