Waste septic Diagnosis?

Facts: 2 story home w/plastic waste pipes (black) to a septic tank. 2 2nd floor comodes and showers, 1 mail floor comode. Washer, etc.
Already done: inspection of 1st floor comode drain to main line clear. Septic tanks pumped clean last week, Main line(20') reamed and videoed clean last week to tank. Main line has a dip about 6" half-way through (----._.----->tank) one of the main stack pipes has a crack where a tee is moulded into it that will leak when pressurized
Symptoms: about 1/2-3/4 of the time the when an upstairs shower is in use or comode flushed the 1st floor comode will bubble. That signals a problem then we get leaking in the basement stack.
Questions:
What might be causin the bubbling?
Other than gutting and rebuilding the stack is there anything that can be done to seal the crack? Its right in the curve where the tee is moulded into the mainl part so using waterproof tape is not really an option.
There are six feeds coming into the stack in in between the joists. Any ideas what a typical cost would be to remove/rebuild this stack?
Last but not least ... should we simply build an outhouse out back?
Thanks for any ideas/suggestions for a frustrated hownowner.
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snipped-for-privacy@fdcx.net wrote:

Clogged or inadequate vent.
Jim
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Yep,
Harry K
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On Feb 27, 6:40 am, snipped-for-privacy@fdcx.net wrote:

Get up on the roof with a clean out tool and go after the debris in the vent stack. Some folks do it with a garden hose. Check the archives here for more.
Joe
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