OT Mystery Plumbing situation

Mystery Plumbing situation with slab on grade rental stoppage.
From the 4" main line outside I guess a 3" or possibly 4" sewer pipe
extends into the house -about 4ft to a minimal size ptrap shower tub drain (1.25 " dia possibly 1.5") the house sewer pipe extends further about 4ft and connect to a toilet and 6' further onto a sink. Either before or after that there is a 4" diameter asbestos vertical roof vent through the roof.
Suddenly - instantly (the tenants, with a 2 year old, says) the toilet failed to drain and some of the brown/black water came up through the shower tub drain but not much. The sink had no similar problems. (The kitchen sink is on a different circuit.)
Neither would drain! But the sink at the far end had no drain problems. And the toilet flush full of water would have different water levels by about 18 inches after each effort!
First I used a toilet auger (has a 36" reach) - that changed nothing. Then I ran a snake about 6 ft into the tub overflow hole - also change nothing. With great difficulty same thing in the toilet.
Water was at two levels and still not draining.
As a desperation move I climbed onto the roof and inserted a garden hose into the vent pipe and gave it a full blast of 90 psi water. This must have scared the s*** out of everything because suddenly everything drained properly! This was no doubt constructed to minimum UPC standards near Los Angeles years ago.
To me - clearly some kind of foreign object put into the toilet - by some human. But what - with the two water levels? The main line had no problems - checked first thing. Could it have been years of accumulation of sewer debris that suddenly got loose and caused this?
Is there a reasonable explanation for any of this and the persistent two water levels? Some of the really brilliant practical minds are known to frequent this news group.
Thanks in advance, mic
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On 8/26/2014 12:30 PM, Mikie wrote:

I don't have a clue about your problem, but I once got to remove about 40 kotex from a 4" cast iron pipe on one of my rentals. The gorgeous lady of the house was not available for comment. Mikek
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Niece's husband had to call a plumber for their rent house. They rent it to UT students. Sewer was plugged with condoms. The kids paid because he photographed the evidence and promised to notify their folks.
That guy is so technically illiterate, my niece has to put all the toys together for Christmas.
Pete Keillor
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wrote:

I have those up to 6" - there always seem to be a vent - of some kind - that relieves the pressure. (Which is all understandable)
I fail to see the logic in two water levels that remained DIFFERENT for a full hour while snaking. I will no doubt need to use it on my own house where a toilet is "sucking" air. Then again I kind of like that it is sucking clear every time - with a 5 gal tank.
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On Tue, 26 Aug 2014 10:30:23 -0700, Mikie wrote:

I'm stuck on "toilet" and "two year old". Did you ask them if they're missing any shoes?
--

Tim Wescott
Wescott Design Services
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On Tue, 26 Aug 2014 17:53:53 -0500, Tim Wescott

Tenants claim to be as pure as the driven snow. Assuming your suggestion is the case - then how can one explain the persistent different water levels?
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On Tue, 26 Aug 2014 16:09:29 -0700, Mikie wrote:

I can't.
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Tim Wescott
Wescott Design Services
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"Either before or after"? C'mon, you can do better than that.

^^^^^^^^^^ You've answered the cause of your problem there.

Perhaps the air-lock caused by the blockage prevented the higher water in the sink from equalizing into the toilet.
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wherever he may be placed, the man who thinks becomes a light and a power.
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