Basement floor drain and utility sink will bubble then back up when kitchen sink is drained or washing machine empties.
We have a small single story ranch built in 1952 (lived in since ’93) with original cast iron sewer plumbing. The kitchen is directly above the laundry room and drains into a 2-1/2” OD heading down to the laundry room and vents to the roof. The utility sink drains into this line (1 ft. above floor) as well as a 36” stand pipe for the washer tapped into the cleanout just above the concrete. The floor drain (with trap and cleanout) in the laundry room also Ys into this line then heads to the center of the house and Ys to the 3-1/2” OD main line about 1 ft. from the cleanout.
I ran a 1/2"x 50 ft power snake with 1-1/4” dia. twin blade cutter from the utility sink Y, the floor drain cleanout and the main line cleanout. I also ran hot water down the sewer vent above the kitchen to verify it was not plugged. All this helped but still getting bubbles then backs up slightly, it does go back down quickly after the sink or washer empties. In years past, this method used to solve the problem but has become less effective in recent years. When draining and looking into the main line with a mirror I can see the drainage flowing freely into the empty main, thus do not think there is any problem with the main.
Thank you in advance for any input or advice.