I did a DIY replacement of an electronic gas valve in my aging trane furnace last year. I used rector seal on all the joints, and did a gas leak check using soapy water(all passed). The furnace operated for a few weeks before the weather started getting warmer with no issues. Fast forward to this winter and I awoke this morning to the faint smell of gas in the house. I turned off the unit and the gas and performed another soap and water leak check (switching gas on briefly for the test). Sure enough, one of the joints is now bubbling. The rector seal appears to be cracked on the outside. Any suggestions as to what went wrong so that I can avoid repeating my mistake?