I would be looking at the tube real close to see if it is bent or cracked at the groove also check and make sure that the tube is going in the case deep enough.
We install tubes at work on a daily basis with NO sealer and rarely ever have one leak if all is in good shape

70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4