Plumbing Conundrum?

[PedroOhare]

In my kitchen i have a two basin sink. The left side has a garbage disposal. Last night while running water, it backed up into the other sink. Each side has a trap an then the drain lines join together in a Y fitting where the go to the basement and down to the main stack. Between the two 'limbs' of the Y there is a removable plug the feeds directly down the joined upright. Thinking that the plug had to be below the Y since both sinks were backed up i run a fifteen foot snake as far as it will go. No luck. So I am proactive and check and clean both traps and the pipes all the way to the Y on both sides. All clear. So thinking maybe the snake isn't quite long enough, I cut the pvc pipe in the basement and run the snake back up the pipe. No blocks (but a bunch of crud). There is no way the snake from either end didn't overlap, but both sinks are still backing up evenly.

My only new thought is that perhaps the vent stack is blocked? Either that or the snake is getting deflected 'up' the vent stack from both ends and the block is at that junction? It would make sense that the snake from the basement would go up the stack, but the path from the sink would require a greater than 90 degree bend as the junction begins to curve down at the vent stack junction.

So would a blocked vent cause the symptoms i describe? An airlock? I could go up in the roof and see if there is anything obvious, but if not I am into cutting through plaster walls just for access? Am I way off or is there a simpler possibility I am missing?

Thanks for any help.

Pedro

[EatenByLimestone]

If the vent was blocked, I'd expect you'd still see a slow drain as gravity will still do its thing.

With the line cut, run a hose into the lower end and see if it backs up. If it is clear, time to test the upper run.


Pour a bucket of water down the drain from the sink. If the water drains without issue, your line is clear indicating the vent is the issue since you proved the 2 runs of actual drain were clear. With the bottom open to let the air out, it should flow quickly, even if the vent is clogged. If the water doesn't flow out, you have a clogged line.