I have just moved into a condo built in 1974 that has a double kitchen sink with disposal. The water drains fine through both drains when running the water or draining a filled sink, but when I run the disposal, it backs up in the other sink. I have a dishwasher that is seldom used connected to the disposal.
Each sink has it's own trap running to a stub into the wall and I assume connected to a Y behind the wall. There does not appear to be a clog as the sink drains well if not using the disposal.
Could the pitch or backfill be wrong?
Thanks for any help.

Hi Tim,

You have a partial clog in the lateral(horizontal)drain line in the wall. The disposal acts like a pump discharging with much pressure and volume the when you just drain the sink. To repair,remove the trap and send a snake on intov the wall. You'll hit a bend about 6" in but work and crank around it and put out about 5' more. Afterwards flush the line out with boiling water. Good luck, Tom