WELL....the plumbing repair didn't work out. My wife started doing laundry last week, and the water started bubbling up again.
We had our plumber friend come over, and he installed another clean-out tee at the garage, and had a friend of his come over with the BIG power snake, and snake the line from the garage to past where it tees into the drain from the house.
They didn't really find any obstructions, and the line is clear from the garage to the street.
Final diagnosis is that the pipe is running up hill from the garage to the house clean-out tee, and water doesn't run uphill very well. It probably got in this condition from land movement and earthquakes over the seventy years since the pipe was installed when they built the house in 1942.
SO......about half the digging is done, and there's another 20 or so feet to go to get to the junction. Monday morning they'll be back out here to finish digging out the old clay-pipe line, and replace it with new ABS pipe, with the correct downhill slope to it, and backfill the trench.
And I was just getting the back yard to look decent.
At least we're getting a big break on the cost of the job, which usually runs about $100/linear foot of pipe replaced.
Since these guys are friends of out good friend, the whole job will "only" cost $1500 instead of $4000 for 40 feet of line replaced.
Oh, the joys of being a homeowner!